docs: def _validate_molecule_env_var(self, molecule_env_var, field, value):
          """ Readonly but with a custom error.
  
          The rule's arguments are validated against this schema:
          {'type': 'boolean'}
          """
          # TODO(retr0h): This needs to be better handled.
          pattern = r'^[{$]+MOLECULE[_a-z0-9A-Z]+[}]*$'
  
          if molecule_env_var:
              if re.match(pattern, value):
                  msg = ('cannot reference $MOLECULE special variables '
                         'in this section')
                  self._error(field, msg)