code: def p_element_list(self, p):
          
          if len(p) == 3:
              p[0] = p[1] + [p[2]]
          else:
              p[1].extend(p[3])
              p[1].append(p[4])
              p[0] = p[1]
docs: element_list : elision_opt assignment_expr
                          | element_list COMMA elision_opt assignment_expr