code: def addDerivesFrom(self, child_id, parent_id):
          
  
          self.graph.addTriple(
              child_id, self.globaltt['derives_from'], parent_id)
  
          return
docs: We add a derives_from relationship between the child and parent id.
          Examples of uses include between:
          an allele and a construct or strain here,
          a cell line and it's parent genotype.  Adding the parent and child to
          the graph should happen outside of this function call to ensure graph
          integrity.
          :param child_id:
          :param parent_id:
          :return: