code: def _add_stmt_edge(self, stmt):
          
          # Skip statements with None in the subject position
          source = _get_node_key(stmt.agent_list()[0])
          target = _get_node_key(stmt.agent_list()[1])
          edge_key = (source, target, stmt.__class__.__name__)
          if edge_key in self.existing_edges:
              return
          self.existing_edges.append(edge_key)
          if isinstance(stmt, RemoveModification) or \
               isinstance(stmt, Inhibition) or \
               isinstance(stmt, DecreaseAmount) or \
               isinstance(stmt, Gap) or \
               (isinstance(stmt, Influence) and stmt.overall_polarity() == -1):
              color = '#ff0000'
          else:
              color = '#000000'
          params = {'color': color,
                    'arrowhead': 'normal',
                    'dir': 'forward'}
          self._add_edge(source, target, **params)
docs: Assemble a Modification statement.