code: def setSubgraphVal(self,  parent_name,  graph_name, field_name, val):
          
          subgraph = self._getSubGraph(parent_name, graph_name, True)
          if subgraph.hasField(field_name):
              subgraph.setVal(field_name, val)
          else:
              raise AttributeError("Invalid field name %s for subgraph %s "
                                   "of parent graph %s." 
                                   % (field_name, graph_name, parent_name))
docs: Set Value for Field in Subgraph.
  
          The private method is for use in retrieveVals() method of child
          classes.
          
          @param parent_name: Root Graph Name
          @param graph_name:  Subgraph Name
          @param field_name:  Field Name.
          @param val:         Value for field.