def METHOD_NAME(G, nbunch1, nbunch2=None):
    """Returns the node boundary of `nbunch1`.
    The *node boundary* of a set *S* with respect to a set *T* is the
    set of nodes *v* in *T* such that for some *u* in *S*, there is an
    edge joining *u* to *v*. If *T* is not specified, it is assumed to
    be the set of all nodes not in *S*.
    Parameters
    ----------
    G : NetworkX graph
    nbunch1 : iterable
        Iterable of nodes in the graph representing the set of nodes
        whose node boundary will be returned. (This is the set *S* from
        the definition above.)
    nbunch2 : iterable
        Iterable of nodes representing the target (or "exterior") set of
        nodes. (This is the set *T* from the definition above.) If not
        specified, this is assumed to be the set of all nodes in `G`
        not in `nbunch1`.
    Returns
    -------
    set
        The node boundary of `nbunch1` with respect to `nbunch2`.
    Examples
    --------
    >>> G = nx.wheel_graph(6)
    When nbunch2=None:
    >>> list(nx.node_boundary(G, (3, 4)))
    [0, 2, 5]
    When nbunch2 is given:
    >>> list(nx.node_boundary(G, (3, 4), (0, 1, 5)))
    [0, 5]
    Notes
    -----
    Any element of `nbunch` that is not in the graph `G` will be
    ignored.
    `nbunch1` and `nbunch2` are usually meant to be disjoint, but in
    the interest of speed and generality, that is not required here.
    """
    nset1 = {n for n in nbunch1 if n in G}
    bdy = set(chain.from_iterable(G[v] for v in nset1)) - nset1
    # If `nbunch2` is not specified, it is assumed to be the set
    # complement of `nbunch1`.
    if nbunch2 is not None:
        bdy &= set(nbunch2)
    return bdy