lmap.is_memoized(k) yields True if k is a key in the given lazy map lmap that is both lazy
        and already memoized.

def is_memoized(self, k):
        '''
        lmap.is_memoized(k) yields True if k is a key in the given lazy map lmap that is both lazy
        and already memoized.
        '''
        v = ps.PMap.__getitem__(self, k)
        if not isinstance(v, (types.FunctionType, partial)):
            return False
        else:
            return id(v) in self._memoized