docs: def get_least_common_subsumer(self,from_tid,to_tid):
          """
          Returns the deepest common subsumer among two terms
          @type from_tid: string
          @param from_tid: one term id
          @type to_tid: string
          @param to_tid: another term id
          @rtype: string
          @return: the term identifier of the common subsumer
          """
          termid_from = self.terminal_for_term.get(from_tid)
          termid_to = self.terminal_for_term.get(to_tid)
          
          path_from = self.paths_for_terminal[termid_from][0]
          path_to = self.paths_for_terminal[termid_to][0]
          common_nodes = set(path_from) & set(path_to)
          if len(common_nodes) == 0:
              return None
          else:
              indexes = []
              for common_node in common_nodes:
                  index1 = path_from.index(common_node)
                  index2 = path_to.index(common_node)
                  indexes.append((common_node,index1+index2))
              indexes.sort(key=itemgetter(1))
              shortest_common = indexes[0][0]
              return shortest_common