code: def variables(self):
          '''return a list of available variables'''
          return sorted(list(self.mapping.vars.keys()),
                        key = lambda v : self.mapping.vars[v].indexf variables(self):
          '''return a list of available variables'''
          return sorted(list(self.mapping.vars.keys()),
                        key = lambda v : self.mapping.vars[v].index)
docs: return a list of available variables