def write(self, file_name, delim=',', sep='\t'):
        """Write a directed hypergraph to a file, where nodes are
        represented as strings.
        Each column is separated by "sep", and the individual
        tail nodes and head nodes are delimited by "delim".
        The header line is currently ignored, but columns should be of
        the format:
        tailnode1[delim]..tailnodeM[sep]headnode1[delim]..headnodeN[sep]weight

        As a concrete example, an arbitrary line with delim=',' and
        sep='    ' (4 spaces) may look like:
        ::

            x1,x2    x3,x4,x5    12

        which defines a hyperedge of weight 12 from a tail set containing
        nodes "x1" and "x2" to a head set containing nodes "x3", "x4", and "x5"

        """
        out_file = open(file_name, 'w')

        # write first header line
        out_file.write("tail" + sep + "head" + sep + "weight\n")

        for hyperedge_id in self.get_hyperedge_id_set():
            line = ""
            # Write each tail node to the line, separated by delim
            for tail_node in self.get_hyperedge_tail(hyperedge_id):
                line += tail_node + delim
            # Remove last (extra) delim
            line = line[:-1]

            # Add sep between columns
            line += sep

            # Write each head node to the line, separated by delim
            for head_node in self.get_hyperedge_head(hyperedge_id):
                line += head_node + delim
            # Remove last (extra) delim
            line = line[:-1]

            # Write the weight to the line and end the line
            line += sep + str(self.get_hyperedge_weight(hyperedge_id)) + "\n"

            out_file.write(line)

        out_file.close()