Readonly but with a custom error.

        The rule's arguments are validated against this schema:
        {'type': 'boolean'}

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)