def _adjacent_verts(facets, vert):
    """Find the adjacent vertices in a hull to the given vertex.

    Args:
        facets (list of sets of ints): Convex hull facets, each item represents
            a facet, with the contents of the set being its vertex indices.
        vert (int): Vertex index to find vertices adjacent to.

    Returns:
        set of ints: Vertices adjacent to `vert`.
    """
    return (set.union(*[facet_verts - set([vert])
                        for facet_verts in facets
                        if vert in facet_verts]) -
            set([vert]))