<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def is_constraint(self):
 """ returns the expression of the constraint that constrains this parameter """

        if self._is_constraint is None:
            return None
        return self._bundle.get_parameter(context='constraint', uniqueid=self._is_constraint)