<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def kwargs(self):
 """The dictionary of keyword-only arguments for the instantiation of the Expression"""

        # Subclasses must override this property if and only if they define
        # keyword-only arguments in their __init__ method
        if hasattr(self, '_has_kwargs') and self._has_kwargs:
            raise NotImplementedError(
                "Class %s does not provide a kwargs property"
                % str(self.__class__.__name__))
        return {}