code: def create_variable_keys(self):
          
          variable_name = '{}_keys'.format(self._variable)
          if variable_name in self._variables:
              return
          self._variables.add(variable_name)
          self.l('{variable}_keys = set({variable}.keys())')
docs: Append code for creating variable with keys of that variable (dictionary)
          with a name ``{variable}_keys``. Similar to `create_variable_with_length`.