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+"""
+Algorithm for testing d-separation in DAGs.
+
+*d-separation* is a test for conditional independence in probability
+distributions that can be factorized using DAGs.  It is a purely
+graphical test that uses the underlying graph and makes no reference
+to the actual distribution parameters.  See [1]_ for a formal
+definition.
+
+The implementation is based on the conceptually simple linear time
+algorithm presented in [2]_.  Refer to [3]_, [4]_ for a couple of
+alternative algorithms.
+
+The functional interface in NetworkX consists of three functions:
+
+- `find_minimal_d_separator` returns a minimal d-separator set ``z``.
+  That is, removing any node or nodes from it makes it no longer a d-separator.
+- `is_d_separator` checks if a given set is a d-separator.
+- `is_minimal_d_separator` checks if a given set is a minimal d-separator.
+
+D-separators
+------------
+
+Here, we provide a brief overview of d-separation and related concepts that
+are relevant for understanding it:
+
+The ideas of d-separation and d-connection relate to paths being open or blocked.
+
+- A "path" is a sequence of nodes connected in order by edges. Unlike for most
+  graph theory analysis, the direction of the edges is ignored. Thus the path
+  can be thought of as a traditional path on the undirected version of the graph.
+- A "candidate d-separator" ``z`` is a set of nodes being considered as
+  possibly blocking all paths between two prescribed sets ``x`` and ``y`` of nodes.
+  We refer to each node in the candidate d-separator as "known".
+- A "collider" node on a path is a node that is a successor of its two neighbor
+  nodes on the path. That is, ``c`` is a collider if the edge directions
+  along the path look like ``... u -> c <- v ...``.
+- If a collider node or any of its descendants are "known", the collider
+  is called an "open collider". Otherwise it is a "blocking collider".
+- Any path can be "blocked" in two ways. If the path contains a "known" node
+  that is not a collider, the path is blocked. Also, if the path contains a
+  collider that is not a "known" node, the path is blocked.
+- A path is "open" if it is not blocked. That is, it is open if every node is
+  either an open collider or not a "known". Said another way, every
+  "known" in the path is a collider and every collider is open (has a
+  "known" as a inclusive descendant). The concept of "open path" is meant to
+  demonstrate a probabilistic conditional dependence between two nodes given
+  prescribed knowledge ("known" nodes).
+- Two sets ``x`` and ``y`` of nodes are "d-separated" by a set of nodes ``z``
+  if all paths between nodes in ``x`` and nodes in ``y`` are blocked. That is,
+  if there are no open paths from any node in ``x`` to any node in ``y``.
+  Such a set ``z`` is a "d-separator" of ``x`` and ``y``.
+- A "minimal d-separator" is a d-separator ``z`` for which no node or subset
+  of nodes can be removed with it still being a d-separator.
+
+The d-separator blocks some paths between ``x`` and ``y`` but opens others.
+Nodes in the d-separator block paths if the nodes are not colliders.
+But if a collider or its descendant nodes are in the d-separation set, the
+colliders are open, allowing a path through that collider.
+
+Illustration of D-separation with examples
+------------------------------------------
+
+A pair of two nodes, ``u`` and ``v``, are d-connected if there is a path
+from ``u`` to ``v`` that is not blocked. That means, there is an open
+path from ``u`` to ``v``.
+
+For example, if the d-separating set is the empty set, then the following paths are
+open between ``u`` and ``v``:
+
+- u <- n -> v
+- u -> w -> ... -> n -> v
+
+If  on the other hand, ``n`` is in the d-separating set, then ``n`` blocks
+those paths between ``u`` and ``v``.
+
+Colliders block a path if they and their descendants are not included
+in the d-separating set. An example of a path that is blocked when the
+d-separating set is empty is:
+
+- u -> w -> ... -> n <- v
+
+The node ``n`` is a collider in this path and is not in the d-separating set.
+So ``n`` blocks this path. However, if ``n`` or a descendant of ``n`` is
+included in the d-separating set, then the path through the collider
+at ``n`` (... -> n <- ...) is "open".
+
+D-separation is concerned with blocking all paths between nodes from ``x`` to ``y``.
+A d-separating set between ``x`` and ``y`` is one where all paths are blocked.
+
+D-separation and its applications in probability
+------------------------------------------------
+
+D-separation is commonly used in probabilistic causal-graph models. D-separation
+connects the idea of probabilistic "dependence" with separation in a graph. If
+one assumes the causal Markov condition [5]_, (every node is conditionally
+independent of its non-descendants, given its parents) then d-separation implies
+conditional independence in probability distributions.
+Symmetrically, d-connection implies dependence.
+
+The intuition is as follows. The edges on a causal graph indicate which nodes
+influence the outcome of other nodes directly. An edge from u to v
+implies that the outcome of event ``u`` influences the probabilities for
+the outcome of event ``v``. Certainly knowing ``u`` changes predictions for ``v``.
+But also knowing ``v`` changes predictions for ``u``. The outcomes are dependent.
+Furthermore, an edge from ``v`` to ``w`` would mean that ``w`` and ``v`` are dependent
+and thus that ``u`` could indirectly influence ``w``.
+
+Without any knowledge about the system (candidate d-separating set is empty)
+a causal graph ``u -> v -> w`` allows all three nodes to be dependent. But
+if we know the outcome of ``v``, the conditional probabilities of outcomes for
+``u`` and ``w`` are independent of each other. That is, once we know the outcome
+for ```v`, the probabilities for ``w`` do not depend on the outcome for ``u``.
+This is the idea behind ``v`` blocking the path if it is "known" (in the candidate
+d-separating set).
+
+The same argument works whether the direction of the edges are both
+left-going and when both arrows head out from the middle. Having a "known"
+node on a path blocks the collider-free path because those relationships
+make the conditional probabilities independent.
+
+The direction of the causal edges does impact dependence precisely in the
+case of a collider e.g. ``u -> v <- w``. In that situation, both ``u`` and ``w``
+influence ``v```. But they do not directly influence each other. So without any
+knowledge of any outcomes, ``u`` and ``w`` are independent. That is the idea behind
+colliders blocking the path. But, if ``v`` is known, the conditional probabilities
+of ``u`` and ``w`` can be dependent. This is the heart of Berkson's Paradox [6]_.
+For example, suppose ``u`` and ``w`` are boolean events (they either happen or do not)
+and ``v`` represents the outcome "at least one of ``u`` and ``w`` occur". Then knowing
+``v`` is true makes the conditional probabilities of ``u`` and ``w`` dependent.
+Essentially, knowing that at least one of them is true raises the probability of
+each. But further knowledge that ``w`` is true (or false) change the conditional
+probability of ``u`` to either the original value or 1. So the conditional
+probability of ``u`` depends on the outcome of ``w`` even though there is no
+causal relationship between them. When a collider is known, dependence can
+occur across paths through that collider. This is the reason open colliders
+do not block paths.
+
+Furthermore, even if ``v`` is not "known", if one of its descendants is "known"
+we can use that information to know more about ``v`` which again makes
+``u`` and ``w`` potentially dependent. Suppose the chance of ``n`` occurring
+is much higher when ``v`` occurs ("at least one of ``u`` and ``w`` occur").
+Then if we know ``n`` occurred, it is more likely that ``v`` occurred and that
+makes the chance of ``u`` and ``w`` dependent. This is the idea behind why
+a collider does no block a path if any descendant of the collider is "known".
+
+When two sets of nodes ``x`` and ``y`` are d-separated by a set ``z``,
+it means that given the outcomes of the nodes in ``z``, the probabilities
+of outcomes of the nodes in ``x`` are independent of the outcomes of the
+nodes in ``y`` and vice versa.
+
+Examples
+--------
+A Hidden Markov Model with 5 observed states and 5 hidden states
+where the hidden states have causal relationships resulting in
+a path results in the following causal network. We check that
+early states along the path are separated from late state in
+the path by the d-separator of the middle hidden state.
+Thus if we condition on the middle hidden state, the early
+state probabilities are independent of the late state outcomes.
+
+>>> G = nx.DiGraph()
+>>> G.add_edges_from(
+...     [
+...         ("H1", "H2"),
+...         ("H2", "H3"),
+...         ("H3", "H4"),
+...         ("H4", "H5"),
+...         ("H1", "O1"),
+...         ("H2", "O2"),
+...         ("H3", "O3"),
+...         ("H4", "O4"),
+...         ("H5", "O5"),
+...     ]
+... )
+>>> x, y, z = ({"H1", "O1"}, {"H5", "O5"}, {"H3"})
+>>> nx.is_d_separator(G, x, y, z)
+True
+>>> nx.is_minimal_d_separator(G, x, y, z)
+True
+>>> nx.is_minimal_d_separator(G, x, y, z | {"O3"})
+False
+>>> z = nx.find_minimal_d_separator(G, x | y, {"O2", "O3", "O4"})
+>>> z == {"H2", "H4"}
+True
+
+If no minimal_d_separator exists, `None` is returned
+
+>>> other_z = nx.find_minimal_d_separator(G, x | y, {"H2", "H3"})
+>>> other_z is None
+True
+
+
+References
+----------
+
+.. [1] Pearl, J.  (2009).  Causality.  Cambridge: Cambridge University Press.
+
+.. [2] Darwiche, A.  (2009).  Modeling and reasoning with Bayesian networks.
+   Cambridge: Cambridge University Press.
+
+.. [3] Shachter, Ross D. "Bayes-ball: The rational pastime (for
+   determining irrelevance and requisite information in belief networks
+   and influence diagrams)." In Proceedings of the Fourteenth Conference
+   on Uncertainty in Artificial Intelligence (UAI), (pp. 480–487). 1998.
+
+.. [4] Koller, D., & Friedman, N. (2009).
+   Probabilistic graphical models: principles and techniques. The MIT Press.
+
+.. [5] https://en.wikipedia.org/wiki/Causal_Markov_condition
+
+.. [6] https://en.wikipedia.org/wiki/Berkson%27s_paradox
+
+"""
+
+from collections import deque
+from itertools import chain
+
+import networkx as nx
+from networkx.utils import UnionFind, not_implemented_for
+
+__all__ = [
+    "is_d_separator",
+    "is_minimal_d_separator",
+    "find_minimal_d_separator",
+    "d_separated",
+    "minimal_d_separator",
+]
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable
+def is_d_separator(G, x, y, z):
+    """Return whether node sets `x` and `y` are d-separated by `z`.
+
+    Parameters
+    ----------
+    G : nx.DiGraph
+        A NetworkX DAG.
+
+    x : node or set of nodes
+        First node or set of nodes in `G`.
+
+    y : node or set of nodes
+        Second node or set of nodes in `G`.
+
+    z : node or set of nodes
+        Potential separator (set of conditioning nodes in `G`). Can be empty set.
+
+    Returns
+    -------
+    b : bool
+        A boolean that is true if `x` is d-separated from `y` given `z` in `G`.
+
+    Raises
+    ------
+    NetworkXError
+        The *d-separation* test is commonly used on disjoint sets of
+        nodes in acyclic directed graphs.  Accordingly, the algorithm
+        raises a :exc:`NetworkXError` if the node sets are not
+        disjoint or if the input graph is not a DAG.
+
+    NodeNotFound
+        If any of the input nodes are not found in the graph,
+        a :exc:`NodeNotFound` exception is raised
+
+    Notes
+    -----
+    A d-separating set in a DAG is a set of nodes that
+    blocks all paths between the two sets. Nodes in `z`
+    block a path if they are part of the path and are not a collider,
+    or a descendant of a collider. Also colliders that are not in `z`
+    block a path. A collider structure along a path
+    is ``... -> c <- ...`` where ``c`` is the collider node.
+
+    https://en.wikipedia.org/wiki/Bayesian_network#d-separation
+    """
+    try:
+        x = {x} if x in G else x
+        y = {y} if y in G else y
+        z = {z} if z in G else z
+
+        intersection = x & y or x & z or y & z
+        if intersection:
+            raise nx.NetworkXError(
+                f"The sets are not disjoint, with intersection {intersection}"
+            )
+
+        set_v = x | y | z
+        if set_v - G.nodes:
+            raise nx.NodeNotFound(f"The node(s) {set_v - G.nodes} are not found in G")
+    except TypeError:
+        raise nx.NodeNotFound("One of x, y, or z is not a node or a set of nodes in G")
+
+    if not nx.is_directed_acyclic_graph(G):
+        raise nx.NetworkXError("graph should be directed acyclic")
+
+    # contains -> and <-> edges from starting node T
+    forward_deque = deque([])
+    forward_visited = set()
+
+    # contains <- and - edges from starting node T
+    backward_deque = deque(x)
+    backward_visited = set()
+
+    ancestors_or_z = set().union(*[nx.ancestors(G, node) for node in x]) | z | x
+
+    while forward_deque or backward_deque:
+        if backward_deque:
+            node = backward_deque.popleft()
+            backward_visited.add(node)
+            if node in y:
+                return False
+            if node in z:
+                continue
+
+            # add <- edges to backward deque
+            backward_deque.extend(G.pred[node].keys() - backward_visited)
+            # add -> edges to forward deque
+            forward_deque.extend(G.succ[node].keys() - forward_visited)
+
+        if forward_deque:
+            node = forward_deque.popleft()
+            forward_visited.add(node)
+            if node in y:
+                return False
+
+            # Consider if -> node <- is opened due to ancestor of node in z
+            if node in ancestors_or_z:
+                # add <- edges to backward deque
+                backward_deque.extend(G.pred[node].keys() - backward_visited)
+            if node not in z:
+                # add -> edges to forward deque
+                forward_deque.extend(G.succ[node].keys() - forward_visited)
+
+    return True
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable
+def find_minimal_d_separator(G, x, y, *, included=None, restricted=None):
+    """Returns a minimal d-separating set between `x` and `y` if possible
+
+    A d-separating set in a DAG is a set of nodes that blocks all
+    paths between the two sets of nodes, `x` and `y`. This function
+    constructs a d-separating set that is "minimal", meaning no nodes can
+    be removed without it losing the d-separating property for `x` and `y`.
+    If no d-separating sets exist for `x` and `y`, this returns `None`.
+
+    In a DAG there may be more than one minimal d-separator between two
+    sets of nodes. Minimal d-separators are not always unique. This function
+    returns one minimal d-separator, or `None` if no d-separator exists.
+
+    Uses the algorithm presented in [1]_. The complexity of the algorithm
+    is :math:`O(m)`, where :math:`m` stands for the number of edges in
+    the subgraph of G consisting of only the ancestors of `x` and `y`.
+    For full details, see [1]_.
+
+    Parameters
+    ----------
+    G : graph
+        A networkx DAG.
+    x : set | node
+        A node or set of nodes in the graph.
+    y : set | node
+        A node or set of nodes in the graph.
+    included : set | node | None
+        A node or set of nodes which must be included in the found separating set,
+        default is None, which means the empty set.
+    restricted : set | node | None
+        Restricted node or set of nodes to consider. Only these nodes can be in
+        the found separating set, default is None meaning all nodes in ``G``.
+
+    Returns
+    -------
+    z : set | None
+        The minimal d-separating set, if at least one d-separating set exists,
+        otherwise None.
+
+    Raises
+    ------
+    NetworkXError
+        Raises a :exc:`NetworkXError` if the input graph is not a DAG
+        or if node sets `x`, `y`, and `included` are not disjoint.
+
+    NodeNotFound
+        If any of the input nodes are not found in the graph,
+        a :exc:`NodeNotFound` exception is raised.
+
+    References
+    ----------
+    .. [1] van der Zander, Benito, and Maciej Liśkiewicz. "Finding
+        minimal d-separators in linear time and applications." In
+        Uncertainty in Artificial Intelligence, pp. 637-647. PMLR, 2020.
+    """
+    if not nx.is_directed_acyclic_graph(G):
+        raise nx.NetworkXError("graph should be directed acyclic")
+
+    try:
+        x = {x} if x in G else x
+        y = {y} if y in G else y
+
+        if included is None:
+            included = set()
+        elif included in G:
+            included = {included}
+
+        if restricted is None:
+            restricted = set(G)
+        elif restricted in G:
+            restricted = {restricted}
+
+        set_y = x | y | included | restricted
+        if set_y - G.nodes:
+            raise nx.NodeNotFound(f"The node(s) {set_y - G.nodes} are not found in G")
+    except TypeError:
+        raise nx.NodeNotFound(
+            "One of x, y, included or restricted is not a node or set of nodes in G"
+        )
+
+    if not included <= restricted:
+        raise nx.NetworkXError(
+            f"Included nodes {included} must be in restricted nodes {restricted}"
+        )
+
+    intersection = x & y or x & included or y & included
+    if intersection:
+        raise nx.NetworkXError(
+            f"The sets x, y, included are not disjoint. Overlap: {intersection}"
+        )
+
+    nodeset = x | y | included
+    ancestors_x_y_included = nodeset.union(*[nx.ancestors(G, node) for node in nodeset])
+
+    z_init = restricted & (ancestors_x_y_included - (x | y))
+
+    x_closure = _reachable(G, x, ancestors_x_y_included, z_init)
+    if x_closure & y:
+        return None
+
+    z_updated = z_init & (x_closure | included)
+    y_closure = _reachable(G, y, ancestors_x_y_included, z_updated)
+    return z_updated & (y_closure | included)
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable
+def is_minimal_d_separator(G, x, y, z, *, included=None, restricted=None):
+    """Determine if `z` is a minimal d-separator for `x` and `y`.
+
+    A d-separator, `z`, in a DAG is a set of nodes that blocks
+    all paths from nodes in set `x` to nodes in set `y`.
+    A minimal d-separator is a d-separator `z` such that removing
+    any subset of nodes makes it no longer a d-separator.
+
+    Note: This function checks whether `z` is a d-separator AND is
+    minimal. One can use the function `is_d_separator` to only check if
+    `z` is a d-separator. See examples below.
+
+    Parameters
+    ----------
+    G : nx.DiGraph
+        A NetworkX DAG.
+    x : node | set
+        A node or set of nodes in the graph.
+    y : node | set
+        A node or set of nodes in the graph.
+    z : node | set
+        The node or set of nodes to check if it is a minimal d-separating set.
+        The function :func:`is_d_separator` is called inside this function
+        to verify that `z` is in fact a d-separator.
+    included : set | node | None
+        A node or set of nodes which must be included in the found separating set,
+        default is ``None``, which means the empty set.
+    restricted : set | node | None
+        Restricted node or set of nodes to consider. Only these nodes can be in
+        the found separating set, default is ``None`` meaning all nodes in ``G``.
+
+    Returns
+    -------
+    bool
+        Whether or not the set `z` is a minimal d-separator subject to
+        `restricted` nodes and `included` node constraints.
+
+    Examples
+    --------
+    >>> G = nx.path_graph([0, 1, 2, 3], create_using=nx.DiGraph)
+    >>> G.add_node(4)
+    >>> nx.is_minimal_d_separator(G, 0, 2, {1})
+    True
+    >>> # since {1} is the minimal d-separator, {1, 3, 4} is not minimal
+    >>> nx.is_minimal_d_separator(G, 0, 2, {1, 3, 4})
+    False
+    >>> # alternatively, if we only want to check that {1, 3, 4} is a d-separator
+    >>> nx.is_d_separator(G, 0, 2, {1, 3, 4})
+    True
+
+    Raises
+    ------
+    NetworkXError
+        Raises a :exc:`NetworkXError` if the input graph is not a DAG.
+
+    NodeNotFound
+        If any of the input nodes are not found in the graph,
+        a :exc:`NodeNotFound` exception is raised.
+
+    References
+    ----------
+    .. [1] van der Zander, Benito, and Maciej Liśkiewicz. "Finding
+        minimal d-separators in linear time and applications." In
+        Uncertainty in Artificial Intelligence, pp. 637-647. PMLR, 2020.
+
+    Notes
+    -----
+    This function works on verifying that a set is minimal and
+    d-separating between two nodes. Uses criterion (a), (b), (c) on
+    page 4 of [1]_. a) closure(`x`) and `y` are disjoint. b) `z` contains
+    all nodes from `included` and is contained in the `restricted`
+    nodes and in the union of ancestors of `x`, `y`, and `included`.
+    c) the nodes in `z` not in `included` are contained in both
+    closure(x) and closure(y). The closure of a set is the set of nodes
+    connected to the set by a directed path in G.
+
+    The complexity is :math:`O(m)`, where :math:`m` stands for the
+    number of edges in the subgraph of G consisting of only the
+    ancestors of `x` and `y`.
+
+    For full details, see [1]_.
+    """
+    if not nx.is_directed_acyclic_graph(G):
+        raise nx.NetworkXError("graph should be directed acyclic")
+
+    try:
+        x = {x} if x in G else x
+        y = {y} if y in G else y
+        z = {z} if z in G else z
+
+        if included is None:
+            included = set()
+        elif included in G:
+            included = {included}
+
+        if restricted is None:
+            restricted = set(G)
+        elif restricted in G:
+            restricted = {restricted}
+
+        set_y = x | y | included | restricted
+        if set_y - G.nodes:
+            raise nx.NodeNotFound(f"The node(s) {set_y - G.nodes} are not found in G")
+    except TypeError:
+        raise nx.NodeNotFound(
+            "One of x, y, z, included or restricted is not a node or set of nodes in G"
+        )
+
+    if not included <= z:
+        raise nx.NetworkXError(
+            f"Included nodes {included} must be in proposed separating set z {x}"
+        )
+    if not z <= restricted:
+        raise nx.NetworkXError(
+            f"Separating set {z} must be contained in restricted set {restricted}"
+        )
+
+    intersection = x.intersection(y) or x.intersection(z) or y.intersection(z)
+    if intersection:
+        raise nx.NetworkXError(
+            f"The sets are not disjoint, with intersection {intersection}"
+        )
+
+    nodeset = x | y | included
+    ancestors_x_y_included = nodeset.union(*[nx.ancestors(G, n) for n in nodeset])
+
+    # criterion (a) -- check that z is actually a separator
+    x_closure = _reachable(G, x, ancestors_x_y_included, z)
+    if x_closure & y:
+        return False
+
+    # criterion (b) -- basic constraint; included and restricted already checked above
+    if not (z <= ancestors_x_y_included):
+        return False
+
+    # criterion (c) -- check that z is minimal
+    y_closure = _reachable(G, y, ancestors_x_y_included, z)
+    if not ((z - included) <= (x_closure & y_closure)):
+        return False
+    return True
+
+
+@not_implemented_for("undirected")
+def _reachable(G, x, a, z):
+    """Modified Bayes-Ball algorithm for finding d-connected nodes.
+
+    Find all nodes in `a` that are d-connected to those in `x` by
+    those in `z`. This is an implementation of the function
+    `REACHABLE` in [1]_ (which is itself a modification of the
+    Bayes-Ball algorithm [2]_) when restricted to DAGs.
+
+    Parameters
+    ----------
+    G : nx.DiGraph
+        A NetworkX DAG.
+    x : node | set
+        A node in the DAG, or a set of nodes.
+    a : node | set
+        A (set of) node(s) in the DAG containing the ancestors of `x`.
+    z : node | set
+        The node or set of nodes conditioned on when checking d-connectedness.
+
+    Returns
+    -------
+    w : set
+        The closure of `x` in `a` with respect to d-connectedness
+        given `z`.
+
+    References
+    ----------
+    .. [1] van der Zander, Benito, and Maciej Liśkiewicz. "Finding
+        minimal d-separators in linear time and applications." In
+        Uncertainty in Artificial Intelligence, pp. 637-647. PMLR, 2020.
+
+    .. [2] Shachter, Ross D. "Bayes-ball: The rational pastime
+       (for determining irrelevance and requisite information in
+       belief networks and influence diagrams)." In Proceedings of the
+       Fourteenth Conference on Uncertainty in Artificial Intelligence
+       (UAI), (pp. 480–487). 1998.
+    """
+
+    def _pass(e, v, f, n):
+        """Whether a ball entering node `v` along edge `e` passes to `n` along `f`.
+
+        Boolean function defined on page 6 of [1]_.
+
+        Parameters
+        ----------
+        e : bool
+            Directed edge by which the ball got to node `v`; `True` iff directed into `v`.
+        v : node
+            Node where the ball is.
+        f : bool
+            Directed edge connecting nodes `v` and `n`; `True` iff directed `n`.
+        n : node
+            Checking whether the ball passes to this node.
+
+        Returns
+        -------
+        b : bool
+            Whether the ball passes or not.
+
+        References
+        ----------
+        .. [1] van der Zander, Benito, and Maciej Liśkiewicz. "Finding
+           minimal d-separators in linear time and applications." In
+           Uncertainty in Artificial Intelligence, pp. 637-647. PMLR, 2020.
+        """
+        is_element_of_A = n in a
+        # almost_definite_status = True  # always true for DAGs; not so for RCGs
+        collider_if_in_Z = v not in z or (e and not f)
+        return is_element_of_A and collider_if_in_Z  # and almost_definite_status
+
+    queue = deque([])
+    for node in x:
+        if bool(G.pred[node]):
+            queue.append((True, node))
+        if bool(G.succ[node]):
+            queue.append((False, node))
+    processed = queue.copy()
+
+    while any(queue):
+        e, v = queue.popleft()
+        preds = ((False, n) for n in G.pred[v])
+        succs = ((True, n) for n in G.succ[v])
+        f_n_pairs = chain(preds, succs)
+        for f, n in f_n_pairs:
+            if (f, n) not in processed and _pass(e, v, f, n):
+                queue.append((f, n))
+                processed.append((f, n))
+
+    return {w for (_, w) in processed}
+
+
+# Deprecated functions:
+def d_separated(G, x, y, z):
+    """Return whether nodes sets ``x`` and ``y`` are d-separated by ``z``.
+
+    .. deprecated:: 3.3
+
+        This function is deprecated and will be removed in NetworkX v3.5.
+        Please use `is_d_separator(G, x, y, z)`.
+
+    """
+    import warnings
+
+    warnings.warn(
+        "d_separated is deprecated and will be removed in NetworkX v3.5."
+        "Please use `is_d_separator(G, x, y, z)`.",
+        category=DeprecationWarning,
+        stacklevel=2,
+    )
+    return nx.is_d_separator(G, x, y, z)
+
+
+def minimal_d_separator(G, u, v):
+    """Returns a minimal_d-separating set between `x` and `y` if possible
+
+    .. deprecated:: 3.3
+
+        minimal_d_separator is deprecated and will be removed in NetworkX v3.5.
+        Please use `find_minimal_d_separator(G, x, y)`.
+
+    """
+    import warnings
+
+    warnings.warn(
+        (
+            "This function is deprecated and will be removed in NetworkX v3.5."
+            "Please use `is_d_separator(G, x, y)`."
+        ),
+        category=DeprecationWarning,
+        stacklevel=2,
+    )
+    return nx.find_minimal_d_separator(G, u, v)