code: def parse_declaration_expressn_operator(self, lhsAST, rhsAST, es, operator):
          
          if isinstance(lhsAST, wdl_parser.Terminal):
              if lhsAST.str == 'string':
                  es = es + '"{string}"'.format(string=lhsAST.source_string)
              else:
                  es = es + '{string}'.format(string=lhsAST.source_string)
          elif isinstance(lhsAST, wdl_parser.Ast):
              es = es + self.parse_declaration_expressn(lhsAST, es='')
          elif isinstance(lhsAST, wdl_parser.AstList):
              raise NotImplementedError
  
          es = es + operator
  
          if isinstance(rhsAST, wdl_parser.Terminal):
              if rhsAST.str == 'string':
                  es = es + '"{string}"'.format(string=rhsAST.source_string)
              else:
                  es = es + '{string}'.format(string=rhsAST.source_string)
          elif isinstance(rhsAST, wdl_parser.Ast):
              es = es + self.parse_declaration_expressn(rhsAST, es='')
          elif isinstance(rhsAST, wdl_parser.AstList):
              raise NotImplementedError
          return es
docs: Simply joins the left and right hand arguments lhs and rhs with an operator.
  
          :param lhsAST:
          :param rhsAST:
          :param es:
          :param operator:
          :return: