def _pre(graph, list_n):
    """Find union of predecessors of nodes in graph defined by C{MultiDiGraph}.

    @param graph: a graph structure corresponding to a FTS.
    @type graph: C{networkx.MultiDiGraph}
    @param list_n: list of nodes whose predecessors need to be returned
    @type list_n: list of nodes in C{graph}
    @return: set of predecessors of C{list_n}
    @rtype: C{set}
    """
    pre_set = set()
    for n in list_n:
        pre_set = pre_set.union(graph.predecessors(n))
    return pre_set