<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_lookup(self, lookup_name):
 """Fetch Lookup by name"""

        from protean.core.repository import BaseLookup
        lookup = self._get_lookup(lookup_name)

        # If unable to find Lookup, or if Lookup is the wrong class, raise Error
        if lookup is None or (lookup is not None and not issubclass(lookup, BaseLookup)):
            raise NotImplementedError

        return lookup