'Return the assymetrical set difference.  That is, the
set of elements in other, except those that are in self.
This is like __sub__ except it handles the case when the left
argument is not a UniSet (but convertible to a UniSet).'

def __rsub__(self, other):

    if (not isinstance(other, UniSet)):
        other = self.fam.c_uniset(other)
    return other.fam.c_sub(other, self)
