def _store_compressed_sequence_to_node(self, node):
        """
        make a compressed representation of a pair of sequences only counting
        the number of times a particular pair of states (e.g. (A,T)) is observed
        the the aligned sequences of parent and child.

        Parameters
        -----------

         node : PhyloTree.Clade
            Tree node. **Note** because the method operates
            on the sequences on both sides of a branch, sequence reconstruction
            must be performed prior to calling this method.

        """
        seq_pairs, multiplicity = self.gtr.compress_sequence_pair(node.up.cseq,
                                              node.cseq,
                                              pattern_multiplicity = self.multiplicity,
                                              ignore_gaps = self.ignore_gaps)
        node.compressed_sequence = {'pair':seq_pairs, 'multiplicity':multiplicity}
make a compressed representation of a pair of sequences only counting
        the number of times a particular pair of states (e.g. (A,T)) is observed
        the the aligned sequences of parent and child.

        Parameters
        -----------

         node : PhyloTree.Clade
            Tree node. **Note** because the method operates
            on the sequences on both sides of a branch, sequence reconstruction
            must be performed prior to calling this method.