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+# $Id: references.py 9613 2024-04-06 13:27:52Z milde $
+# Author: David Goodger <goodger@python.org>
+# Copyright: This module has been placed in the public domain.
+
+"""
+Transforms for resolving references.
+"""
+
+__docformat__ = 'reStructuredText'
+
+from docutils import nodes, utils
+from docutils.transforms import Transform
+
+
+class PropagateTargets(Transform):
+
+    """
+    Propagate empty internal targets to the next element.
+
+    Given the following nodes::
+
+        <target ids="internal1" names="internal1">
+        <target anonymous="1" ids="id1">
+        <target ids="internal2" names="internal2">
+        <paragraph>
+            This is a test.
+
+    `PropagateTargets` propagates the ids and names of the internal
+    targets preceding the paragraph to the paragraph itself::
+
+        <target refid="internal1">
+        <target anonymous="1" refid="id1">
+        <target refid="internal2">
+        <paragraph ids="internal2 id1 internal1" names="internal2 internal1">
+            This is a test.
+    """
+
+    default_priority = 260
+
+    def apply(self):
+        for target in self.document.findall(nodes.target):
+            # Only block-level targets without reference (like ".. _target:"):
+            if (isinstance(target.parent, nodes.TextElement)
+                or (target.hasattr('refid') or target.hasattr('refuri')
+                    or target.hasattr('refname'))):
+                continue
+            assert len(target) == 0, 'error: block-level target has children'
+            next_node = target.next_node(ascend=True)
+            # skip system messages (may be removed by universal.FilterMessages)
+            while isinstance(next_node, nodes.system_message):
+                next_node = next_node.next_node(ascend=True, descend=False)
+            # Do not move names and ids into Invisibles (we'd lose the
+            # attributes) or different Targetables (e.g. footnotes).
+            if (next_node is None
+                or isinstance(next_node, (nodes.Invisible, nodes.Targetable))
+                and not isinstance(next_node, nodes.target)):
+                continue
+            next_node['ids'].extend(target['ids'])
+            next_node['names'].extend(target['names'])
+            # Set defaults for next_node.expect_referenced_by_name/id.
+            if not hasattr(next_node, 'expect_referenced_by_name'):
+                next_node.expect_referenced_by_name = {}
+            if not hasattr(next_node, 'expect_referenced_by_id'):
+                next_node.expect_referenced_by_id = {}
+            for id in target['ids']:
+                # Update IDs to node mapping.
+                self.document.ids[id] = next_node
+                # If next_node is referenced by id ``id``, this
+                # target shall be marked as referenced.
+                next_node.expect_referenced_by_id[id] = target
+            for name in target['names']:
+                next_node.expect_referenced_by_name[name] = target
+            # If there are any expect_referenced_by_... attributes
+            # in target set, copy them to next_node.
+            next_node.expect_referenced_by_name.update(
+                getattr(target, 'expect_referenced_by_name', {}))
+            next_node.expect_referenced_by_id.update(
+                getattr(target, 'expect_referenced_by_id', {}))
+            # Set refid to point to the first former ID of target
+            # which is now an ID of next_node.
+            target['refid'] = target['ids'][0]
+            # Clear ids and names; they have been moved to
+            # next_node.
+            target['ids'] = []
+            target['names'] = []
+            self.document.note_refid(target)
+
+
+class AnonymousHyperlinks(Transform):
+
+    """
+    Link anonymous references to targets.  Given::
+
+        <paragraph>
+            <reference anonymous="1">
+                internal
+            <reference anonymous="1">
+                external
+        <target anonymous="1" ids="id1">
+        <target anonymous="1" ids="id2" refuri="http://external">
+
+    Corresponding references are linked via "refid" or resolved via "refuri"::
+
+        <paragraph>
+            <reference anonymous="1" refid="id1">
+                text
+            <reference anonymous="1" refuri="http://external">
+                external
+        <target anonymous="1" ids="id1">
+        <target anonymous="1" ids="id2" refuri="http://external">
+    """
+
+    default_priority = 440
+
+    def apply(self):
+        anonymous_refs = []
+        anonymous_targets = []
+        for node in self.document.findall(nodes.reference):
+            if node.get('anonymous'):
+                anonymous_refs.append(node)
+        for node in self.document.findall(nodes.target):
+            if node.get('anonymous'):
+                anonymous_targets.append(node)
+        if len(anonymous_refs) != len(anonymous_targets):
+            msg = self.document.reporter.error(
+                  'Anonymous hyperlink mismatch: %s references but %s '
+                  'targets.\nSee "backrefs" attribute for IDs.'
+                  % (len(anonymous_refs), len(anonymous_targets)))
+            msgid = self.document.set_id(msg)
+            for ref in anonymous_refs:
+                prb = nodes.problematic(
+                      ref.rawsource, ref.rawsource, refid=msgid)
+                prbid = self.document.set_id(prb)
+                msg.add_backref(prbid)
+                ref.replace_self(prb)
+            return
+        for ref, target in zip(anonymous_refs, anonymous_targets):
+            target.referenced = 1
+            while True:
+                if target.hasattr('refuri'):
+                    ref['refuri'] = target['refuri']
+                    ref.resolved = 1
+                    break
+                else:
+                    if not target['ids']:
+                        # Propagated target.
+                        target = self.document.ids[target['refid']]
+                        continue
+                    ref['refid'] = target['ids'][0]
+                    self.document.note_refid(ref)
+                    break
+
+
+class IndirectHyperlinks(Transform):
+
+    """
+    a) Indirect external references::
+
+           <paragraph>
+               <reference refname="indirect external">
+                   indirect external
+           <target id="id1" name="direct external"
+               refuri="http://indirect">
+           <target id="id2" name="indirect external"
+               refname="direct external">
+
+       The "refuri" attribute is migrated back to all indirect targets
+       from the final direct target (i.e. a target not referring to
+       another indirect target)::
+
+           <paragraph>
+               <reference refname="indirect external">
+                   indirect external
+           <target id="id1" name="direct external"
+               refuri="http://indirect">
+           <target id="id2" name="indirect external"
+               refuri="http://indirect">
+
+       Once the attribute is migrated, the preexisting "refname" attribute
+       is dropped.
+
+    b) Indirect internal references::
+
+           <target id="id1" name="final target">
+           <paragraph>
+               <reference refname="indirect internal">
+                   indirect internal
+           <target id="id2" name="indirect internal 2"
+               refname="final target">
+           <target id="id3" name="indirect internal"
+               refname="indirect internal 2">
+
+       Targets which indirectly refer to an internal target become one-hop
+       indirect (their "refid" attributes are directly set to the internal
+       target's "id"). References which indirectly refer to an internal
+       target become direct internal references::
+
+           <target id="id1" name="final target">
+           <paragraph>
+               <reference refid="id1">
+                   indirect internal
+           <target id="id2" name="indirect internal 2" refid="id1">
+           <target id="id3" name="indirect internal" refid="id1">
+    """
+
+    default_priority = 460
+
+    def apply(self):
+        for target in self.document.indirect_targets:
+            if not target.resolved:
+                self.resolve_indirect_target(target)
+            self.resolve_indirect_references(target)
+
+    def resolve_indirect_target(self, target):
+        refname = target.get('refname')
+        if refname is None:
+            reftarget_id = target['refid']
+        else:
+            reftarget_id = self.document.nameids.get(refname)
+            if not reftarget_id:
+                # Check the unknown_reference_resolvers
+                for resolver_function in \
+                        self.document.transformer.unknown_reference_resolvers:
+                    if resolver_function(target):
+                        break
+                else:
+                    self.nonexistent_indirect_target(target)
+                return
+        reftarget = self.document.ids[reftarget_id]
+        reftarget.note_referenced_by(id=reftarget_id)
+        if (isinstance(reftarget, nodes.target)
+            and not reftarget.resolved
+            and reftarget.hasattr('refname')):
+            if hasattr(target, 'multiply_indirect'):
+                self.circular_indirect_reference(target)
+                return
+            target.multiply_indirect = 1
+            self.resolve_indirect_target(reftarget)  # multiply indirect
+            del target.multiply_indirect
+        if reftarget.hasattr('refuri'):
+            target['refuri'] = reftarget['refuri']
+            if 'refid' in target:
+                del target['refid']
+        elif reftarget.hasattr('refid'):
+            target['refid'] = reftarget['refid']
+            self.document.note_refid(target)
+        else:
+            if reftarget['ids']:
+                target['refid'] = reftarget_id
+                self.document.note_refid(target)
+            else:
+                self.nonexistent_indirect_target(target)
+                return
+        if refname is not None:
+            del target['refname']
+        target.resolved = 1
+
+    def nonexistent_indirect_target(self, target):
+        if target['refname'] in self.document.nameids:
+            self.indirect_target_error(target, 'which is a duplicate, and '
+                                       'cannot be used as a unique reference')
+        else:
+            self.indirect_target_error(target, 'which does not exist')
+
+    def circular_indirect_reference(self, target):
+        self.indirect_target_error(target, 'forming a circular reference')
+
+    def indirect_target_error(self, target, explanation):
+        naming = ''
+        reflist = []
+        if target['names']:
+            naming = '"%s" ' % target['names'][0]
+        for name in target['names']:
+            reflist.extend(self.document.refnames.get(name, []))
+        for id in target['ids']:
+            reflist.extend(self.document.refids.get(id, []))
+        if target['ids']:
+            naming += '(id="%s")' % target['ids'][0]
+        msg = self.document.reporter.error(
+              'Indirect hyperlink target %s refers to target "%s", %s.'
+              % (naming, target['refname'], explanation), base_node=target)
+        msgid = self.document.set_id(msg)
+        for ref in utils.uniq(reflist):
+            prb = nodes.problematic(
+                  ref.rawsource, ref.rawsource, refid=msgid)
+            prbid = self.document.set_id(prb)
+            msg.add_backref(prbid)
+            ref.replace_self(prb)
+        target.resolved = 1
+
+    def resolve_indirect_references(self, target):
+        if target.hasattr('refid'):
+            attname = 'refid'
+            call_method = self.document.note_refid
+        elif target.hasattr('refuri'):
+            attname = 'refuri'
+            call_method = None
+        else:
+            return
+        attval = target[attname]
+        for name in target['names']:
+            reflist = self.document.refnames.get(name, [])
+            if reflist:
+                target.note_referenced_by(name=name)
+            for ref in reflist:
+                if ref.resolved:
+                    continue
+                del ref['refname']
+                ref[attname] = attval
+                if call_method:
+                    call_method(ref)
+                ref.resolved = 1
+                if isinstance(ref, nodes.target):
+                    self.resolve_indirect_references(ref)
+        for id in target['ids']:
+            reflist = self.document.refids.get(id, [])
+            if reflist:
+                target.note_referenced_by(id=id)
+            for ref in reflist:
+                if ref.resolved:
+                    continue
+                del ref['refid']
+                ref[attname] = attval
+                if call_method:
+                    call_method(ref)
+                ref.resolved = 1
+                if isinstance(ref, nodes.target):
+                    self.resolve_indirect_references(ref)
+
+
+class ExternalTargets(Transform):
+
+    """
+    Given::
+
+        <paragraph>
+            <reference refname="direct external">
+                direct external
+        <target id="id1" name="direct external" refuri="http://direct">
+
+    The "refname" attribute is replaced by the direct "refuri" attribute::
+
+        <paragraph>
+            <reference refuri="http://direct">
+                direct external
+        <target id="id1" name="direct external" refuri="http://direct">
+    """
+
+    default_priority = 640
+
+    def apply(self):
+        for target in self.document.findall(nodes.target):
+            if target.hasattr('refuri'):
+                refuri = target['refuri']
+                for name in target['names']:
+                    reflist = self.document.refnames.get(name, [])
+                    if reflist:
+                        target.note_referenced_by(name=name)
+                    for ref in reflist:
+                        if ref.resolved:
+                            continue
+                        del ref['refname']
+                        ref['refuri'] = refuri
+                        ref.resolved = 1
+
+
+class InternalTargets(Transform):
+
+    default_priority = 660
+
+    def apply(self):
+        for target in self.document.findall(nodes.target):
+            if not target.hasattr('refuri') and not target.hasattr('refid'):
+                self.resolve_reference_ids(target)
+
+    def resolve_reference_ids(self, target):
+        """
+        Given::
+
+            <paragraph>
+                <reference refname="direct internal">
+                    direct internal
+            <target id="id1" name="direct internal">
+
+        The "refname" attribute is replaced by "refid" linking to the target's
+        "id"::
+
+            <paragraph>
+                <reference refid="id1">
+                    direct internal
+            <target id="id1" name="direct internal">
+        """
+        for name in target['names']:
+            refid = self.document.nameids.get(name)
+            reflist = self.document.refnames.get(name, [])
+            if reflist:
+                target.note_referenced_by(name=name)
+            for ref in reflist:
+                if ref.resolved:
+                    continue
+                if refid:
+                    del ref['refname']
+                    ref['refid'] = refid
+                ref.resolved = 1
+
+
+class Footnotes(Transform):
+
+    """
+    Assign numbers to autonumbered footnotes, and resolve links to footnotes,
+    citations, and their references.
+
+    Given the following ``document`` as input::
+
+        <document>
+            <paragraph>
+                A labeled autonumbered footnote reference:
+                <footnote_reference auto="1" id="id1" refname="footnote">
+            <paragraph>
+                An unlabeled autonumbered footnote reference:
+                <footnote_reference auto="1" id="id2">
+            <footnote auto="1" id="id3">
+                <paragraph>
+                    Unlabeled autonumbered footnote.
+            <footnote auto="1" id="footnote" name="footnote">
+                <paragraph>
+                    Labeled autonumbered footnote.
+
+    Auto-numbered footnotes have attribute ``auto="1"`` and no label.
+    Auto-numbered footnote_references have no reference text (they're
+    empty elements). When resolving the numbering, a ``label`` element
+    is added to the beginning of the ``footnote``, and reference text
+    to the ``footnote_reference``.
+
+    The transformed result will be::
+
+        <document>
+            <paragraph>
+                A labeled autonumbered footnote reference:
+                <footnote_reference auto="1" id="id1" refid="footnote">
+                    2
+            <paragraph>
+                An unlabeled autonumbered footnote reference:
+                <footnote_reference auto="1" id="id2" refid="id3">
+                    1
+            <footnote auto="1" id="id3" backrefs="id2">
+                <label>
+                    1
+                <paragraph>
+                    Unlabeled autonumbered footnote.
+            <footnote auto="1" id="footnote" name="footnote" backrefs="id1">
+                <label>
+                    2
+                <paragraph>
+                    Labeled autonumbered footnote.
+
+    Note that the footnotes are not in the same order as the references.
+
+    The labels and reference text are added to the auto-numbered ``footnote``
+    and ``footnote_reference`` elements.  Footnote elements are backlinked to
+    their references via "refids" attributes.  References are assigned "id"
+    and "refid" attributes.
+
+    After adding labels and reference text, the "auto" attributes can be
+    ignored.
+    """
+
+    default_priority = 620
+
+    autofootnote_labels = None
+    """Keep track of unlabeled autonumbered footnotes."""
+
+    symbols = [
+          # Entries 1-4 and 6 below are from section 12.51 of
+          # The Chicago Manual of Style, 14th edition.
+          '*',                          # asterisk/star
+          '\u2020',                    # † &dagger; dagger
+          '\u2021',                    # ‡ &Dagger; double dagger
+          '\u00A7',                    # § &sect; section mark
+          '\u00B6',                    # ¶ &para; paragraph mark (pilcrow)
+                                        # (parallels ['||'] in CMoS)
+          '#',                          # number sign
+          # The entries below were chosen arbitrarily.
+          '\u2660',                    # ♠ &spades; spade suit
+          '\u2665',                    # ♡ &hearts; heart suit
+          '\u2666',                    # ♢ &diams; diamond suit
+          '\u2663',                    # ♣ &clubs; club suit
+          ]
+
+    def apply(self):
+        self.autofootnote_labels = []
+        startnum = self.document.autofootnote_start
+        self.document.autofootnote_start = self.number_footnotes(startnum)
+        self.number_footnote_references(startnum)
+        self.symbolize_footnotes()
+        self.resolve_footnotes_and_citations()
+
+    def number_footnotes(self, startnum):
+        """
+        Assign numbers to autonumbered footnotes.
+
+        For labeled autonumbered footnotes, copy the number over to
+        corresponding footnote references.
+        """
+        for footnote in self.document.autofootnotes:
+            while True:
+                label = str(startnum)
+                startnum += 1
+                if label not in self.document.nameids:
+                    break
+            footnote.insert(0, nodes.label('', label))
+            for name in footnote['names']:
+                for ref in self.document.footnote_refs.get(name, []):
+                    ref += nodes.Text(label)
+                    ref.delattr('refname')
+                    assert len(footnote['ids']) == len(ref['ids']) == 1
+                    ref['refid'] = footnote['ids'][0]
+                    footnote.add_backref(ref['ids'][0])
+                    self.document.note_refid(ref)
+                    ref.resolved = 1
+            if not footnote['names'] and not footnote['dupnames']:
+                footnote['names'].append(label)
+                self.document.note_explicit_target(footnote, footnote)
+                self.autofootnote_labels.append(label)
+        return startnum
+
+    def number_footnote_references(self, startnum):
+        """Assign numbers to autonumbered footnote references."""
+        i = 0
+        for ref in self.document.autofootnote_refs:
+            if ref.resolved or ref.hasattr('refid'):
+                continue
+            try:
+                label = self.autofootnote_labels[i]
+            except IndexError:
+                msg = self.document.reporter.error(
+                      'Too many autonumbered footnote references: only %s '
+                      'corresponding footnotes available.'
+                      % len(self.autofootnote_labels), base_node=ref)
+                msgid = self.document.set_id(msg)
+                for ref in self.document.autofootnote_refs[i:]:
+                    if ref.resolved or ref.hasattr('refname'):
+                        continue
+                    prb = nodes.problematic(
+                          ref.rawsource, ref.rawsource, refid=msgid)
+                    prbid = self.document.set_id(prb)
+                    msg.add_backref(prbid)
+                    ref.replace_self(prb)
+                break
+            ref += nodes.Text(label)
+            id = self.document.nameids[label]
+            footnote = self.document.ids[id]
+            ref['refid'] = id
+            self.document.note_refid(ref)
+            assert len(ref['ids']) == 1
+            footnote.add_backref(ref['ids'][0])
+            ref.resolved = 1
+            i += 1
+
+    def symbolize_footnotes(self):
+        """Add symbols indexes to "[*]"-style footnotes and references."""
+        labels = []
+        for footnote in self.document.symbol_footnotes:
+            reps, index = divmod(self.document.symbol_footnote_start,
+                                 len(self.symbols))
+            labeltext = self.symbols[index] * (reps + 1)
+            labels.append(labeltext)
+            footnote.insert(0, nodes.label('', labeltext))
+            self.document.symbol_footnote_start += 1
+            self.document.set_id(footnote)
+        i = 0
+        for ref in self.document.symbol_footnote_refs:
+            try:
+                ref += nodes.Text(labels[i])
+            except IndexError:
+                msg = self.document.reporter.error(
+                      'Too many symbol footnote references: only %s '
+                      'corresponding footnotes available.' % len(labels),
+                      base_node=ref)
+                msgid = self.document.set_id(msg)
+                for ref in self.document.symbol_footnote_refs[i:]:
+                    if ref.resolved or ref.hasattr('refid'):
+                        continue
+                    prb = nodes.problematic(
+                          ref.rawsource, ref.rawsource, refid=msgid)
+                    prbid = self.document.set_id(prb)
+                    msg.add_backref(prbid)
+                    ref.replace_self(prb)
+                break
+            footnote = self.document.symbol_footnotes[i]
+            assert len(footnote['ids']) == 1
+            ref['refid'] = footnote['ids'][0]
+            self.document.note_refid(ref)
+            footnote.add_backref(ref['ids'][0])
+            i += 1
+
+    def resolve_footnotes_and_citations(self):
+        """
+        Link manually-labeled footnotes and citations to/from their
+        references.
+        """
+        for footnote in self.document.footnotes:
+            for label in footnote['names']:
+                if label in self.document.footnote_refs:
+                    reflist = self.document.footnote_refs[label]
+                    self.resolve_references(footnote, reflist)
+        for citation in self.document.citations:
+            for label in citation['names']:
+                if label in self.document.citation_refs:
+                    reflist = self.document.citation_refs[label]
+                    self.resolve_references(citation, reflist)
+
+    def resolve_references(self, note, reflist):
+        assert len(note['ids']) == 1
+        id = note['ids'][0]
+        for ref in reflist:
+            if ref.resolved:
+                continue
+            ref.delattr('refname')
+            ref['refid'] = id
+            assert len(ref['ids']) == 1
+            note.add_backref(ref['ids'][0])
+            ref.resolved = 1
+        note.resolved = 1
+
+
+class CircularSubstitutionDefinitionError(Exception):
+    pass
+
+
+class Substitutions(Transform):
+
+    """
+    Given the following ``document`` as input::
+
+        <document>
+            <paragraph>
+                The
+                <substitution_reference refname="biohazard">
+                    biohazard
+                 symbol is deservedly scary-looking.
+            <substitution_definition name="biohazard">
+                <image alt="biohazard" uri="biohazard.png">
+
+    The ``substitution_reference`` will simply be replaced by the
+    contents of the corresponding ``substitution_definition``.
+
+    The transformed result will be::
+
+        <document>
+            <paragraph>
+                The
+                <image alt="biohazard" uri="biohazard.png">
+                 symbol is deservedly scary-looking.
+            <substitution_definition name="biohazard">
+                <image alt="biohazard" uri="biohazard.png">
+    """
+
+    default_priority = 220
+    """The Substitutions transform has to be applied very early, before
+    `docutils.transforms.frontmatter.DocTitle` and others."""
+
+    def apply(self):
+        defs = self.document.substitution_defs
+        normed = self.document.substitution_names
+        nested = {}
+        line_length_limit = getattr(self.document.settings,
+                                    "line_length_limit", 10000)
+
+        subreflist = list(self.document.findall(nodes.substitution_reference))
+        for ref in subreflist:
+            msg = ''
+            refname = ref['refname']
+            if refname in defs:
+                key = refname
+            else:
+                normed_name = refname.lower()
+                key = normed.get(normed_name, None)
+            if key is None:
+                msg = self.document.reporter.error(
+                      'Undefined substitution referenced: "%s".'
+                      % refname, base_node=ref)
+            else:
+                subdef = defs[key]
+                if len(subdef.astext()) > line_length_limit:
+                    msg = self.document.reporter.error(
+                            'Substitution definition "%s" exceeds the'
+                            ' line-length-limit.' % key)
+            if msg:
+                msgid = self.document.set_id(msg)
+                prb = nodes.problematic(
+                      ref.rawsource, ref.rawsource, refid=msgid)
+                prbid = self.document.set_id(prb)
+                msg.add_backref(prbid)
+                ref.replace_self(prb)
+                continue
+
+            parent = ref.parent
+            index = parent.index(ref)
+            if ('ltrim' in subdef.attributes
+                or 'trim' in subdef.attributes):
+                if index > 0 and isinstance(parent[index - 1],
+                                            nodes.Text):
+                    parent[index - 1] = parent[index - 1].rstrip()
+            if ('rtrim' in subdef.attributes
+                or 'trim' in subdef.attributes):
+                if (len(parent) > index + 1
+                    and isinstance(parent[index + 1], nodes.Text)):
+                    parent[index + 1] = parent[index + 1].lstrip()
+            subdef_copy = subdef.deepcopy()
+            try:
+                # Take care of nested substitution references:
+                for nested_ref in subdef_copy.findall(
+                        nodes.substitution_reference):
+                    nested_name = normed[nested_ref['refname'].lower()]
+                    if nested_name in nested.setdefault(nested_name, []):
+                        raise CircularSubstitutionDefinitionError
+                    nested[nested_name].append(key)
+                    nested_ref['ref-origin'] = ref
+                    subreflist.append(nested_ref)
+            except CircularSubstitutionDefinitionError:
+                parent = ref.parent
+                if isinstance(parent, nodes.substitution_definition):
+                    msg = self.document.reporter.error(
+                        'Circular substitution definition detected:',
+                        nodes.literal_block(parent.rawsource,
+                                            parent.rawsource),
+                        line=parent.line, base_node=parent)
+                    parent.replace_self(msg)
+                else:
+                    # find original ref substitution which caused this error
+                    ref_origin = ref
+                    while ref_origin.hasattr('ref-origin'):
+                        ref_origin = ref_origin['ref-origin']
+                    msg = self.document.reporter.error(
+                        'Circular substitution definition referenced: '
+                        '"%s".' % refname, base_node=ref_origin)
+                    msgid = self.document.set_id(msg)
+                    prb = nodes.problematic(
+                        ref.rawsource, ref.rawsource, refid=msgid)
+                    prbid = self.document.set_id(prb)
+                    msg.add_backref(prbid)
+                    ref.replace_self(prb)
+                continue
+            ref.replace_self(subdef_copy.children)
+            # register refname of the replacement node(s)
+            # (needed for resolution of references)
+            for node in subdef_copy.children:
+                if isinstance(node, nodes.Referential):
+                    # HACK: verify refname attribute exists.
+                    # Test with docs/dev/todo.txt, see. |donate|
+                    if 'refname' in node:
+                        self.document.note_refname(node)
+
+
+class TargetNotes(Transform):
+
+    """
+    Creates a footnote for each external target in the text, and corresponding
+    footnote references after each reference.
+    """
+
+    default_priority = 540
+    """The TargetNotes transform has to be applied after `IndirectHyperlinks`
+    but before `Footnotes`."""
+
+    def __init__(self, document, startnode):
+        Transform.__init__(self, document, startnode=startnode)
+
+        self.classes = startnode.details.get('class', [])
+
+    def apply(self):
+        notes = {}
+        nodelist = []
+        for target in self.document.findall(nodes.target):
+            # Only external targets.
+            if not target.hasattr('refuri'):
+                continue
+            names = target['names']
+            refs = []
+            for name in names:
+                refs.extend(self.document.refnames.get(name, []))
+            if not refs:
+                continue
+            footnote = self.make_target_footnote(target['refuri'], refs,
+                                                 notes)
+            if target['refuri'] not in notes:
+                notes[target['refuri']] = footnote
+                nodelist.append(footnote)
+        # Take care of anonymous references.
+        for ref in self.document.findall(nodes.reference):
+            if not ref.get('anonymous'):
+                continue
+            if ref.hasattr('refuri'):
+                footnote = self.make_target_footnote(ref['refuri'], [ref],
+                                                     notes)
+                if ref['refuri'] not in notes:
+                    notes[ref['refuri']] = footnote
+                    nodelist.append(footnote)
+        self.startnode.replace_self(nodelist)
+
+    def make_target_footnote(self, refuri, refs, notes):
+        if refuri in notes:  # duplicate?
+            footnote = notes[refuri]
+            assert len(footnote['names']) == 1
+            footnote_name = footnote['names'][0]
+        else:                           # original
+            footnote = nodes.footnote()
+            footnote_id = self.document.set_id(footnote)
+            # Use uppercase letters and a colon; they can't be
+            # produced inside names by the parser.
+            footnote_name = 'TARGET_NOTE: ' + footnote_id
+            footnote['auto'] = 1
+            footnote['names'] = [footnote_name]
+            footnote_paragraph = nodes.paragraph()
+            footnote_paragraph += nodes.reference('', refuri, refuri=refuri)
+            footnote += footnote_paragraph
+            self.document.note_autofootnote(footnote)
+            self.document.note_explicit_target(footnote, footnote)
+        for ref in refs:
+            if isinstance(ref, nodes.target):
+                continue
+            refnode = nodes.footnote_reference(refname=footnote_name, auto=1)
+            refnode['classes'] += self.classes
+            self.document.note_autofootnote_ref(refnode)
+            self.document.note_footnote_ref(refnode)
+            index = ref.parent.index(ref) + 1
+            reflist = [refnode]
+            if not utils.get_trim_footnote_ref_space(self.document.settings):
+                if self.classes:
+                    reflist.insert(
+                        0, nodes.inline(text=' ', Classes=self.classes))
+                else:
+                    reflist.insert(0, nodes.Text(' '))
+            ref.parent.insert(index, reflist)
+        return footnote
+
+
+class DanglingReferences(Transform):
+
+    """
+    Check for dangling references (incl. footnote & citation) and for
+    unreferenced targets.
+    """
+
+    default_priority = 850
+
+    def apply(self):
+        visitor = DanglingReferencesVisitor(
+            self.document,
+            self.document.transformer.unknown_reference_resolvers)
+        self.document.walk(visitor)
+        # *After* resolving all references, check for unreferenced
+        # targets:
+        for target in self.document.findall(nodes.target):
+            if not target.referenced:
+                if target.get('anonymous'):
+                    # If we have unreferenced anonymous targets, there
+                    # is already an error message about anonymous
+                    # hyperlink mismatch; no need to generate another
+                    # message.
+                    continue
+                if target['names']:
+                    naming = target['names'][0]
+                elif target['ids']:
+                    naming = target['ids'][0]
+                else:
+                    # Hack: Propagated targets always have their refid
+                    # attribute set.
+                    naming = target['refid']
+                self.document.reporter.info(
+                    'Hyperlink target "%s" is not referenced.'
+                    % naming, base_node=target)
+
+
+class DanglingReferencesVisitor(nodes.SparseNodeVisitor):
+
+    def __init__(self, document, unknown_reference_resolvers):
+        nodes.SparseNodeVisitor.__init__(self, document)
+        self.document = document
+        self.unknown_reference_resolvers = unknown_reference_resolvers
+
+    def unknown_visit(self, node):
+        pass
+
+    def visit_reference(self, node):
+        if node.resolved or not node.hasattr('refname'):
+            return
+        refname = node['refname']
+        id = self.document.nameids.get(refname)
+        if id is None:
+            for resolver_function in self.unknown_reference_resolvers:
+                if resolver_function(node):
+                    break
+            else:
+                if (getattr(self.document.settings, 'use_bibtex', False)
+                    and isinstance(node, nodes.citation_reference)):
+                    # targets added from BibTeX database by LaTeX
+                    node.resolved = True
+                    return
+                if refname in self.document.nameids:
+                    msg = self.document.reporter.error(
+                        'Duplicate target name, cannot be used as a unique '
+                        'reference: "%s".' % (node['refname']), base_node=node)
+                else:
+                    msg = self.document.reporter.error(
+                        f'Unknown target name: "{node["refname"]}".',
+                        base_node=node)
+                msgid = self.document.set_id(msg)
+                prb = nodes.problematic(
+                      node.rawsource, node.rawsource, refid=msgid)
+                try:
+                    prbid = node['ids'][0]
+                except IndexError:
+                    prbid = self.document.set_id(prb)
+                msg.add_backref(prbid)
+                node.replace_self(prb)
+        else:
+            del node['refname']
+            node['refid'] = id
+            self.document.ids[id].note_referenced_by(id=id)
+            node.resolved = True
+
+    visit_footnote_reference = visit_citation_reference = visit_reference