<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def type(self, value):
 """Set the type of the MOC. The value should be either "IMAGE" or "CATALOG". """


        self._type = None
        if value is None:
            return

        value = value.upper()
        if value in MOC_TYPES:
            self._type = value
        else:
            raise ValueError('MOC type must be one of ' + ', '.join(MOC_TYPES))