'Redefine this object in terms of a polymorphic subclass.
Returns a new PropComparator from which further criterion can be
evaluated.
e.g.::
query.join(Company.employees.of_type(Engineer)).\
filter(Engineer.name==\'foo\')
:param \class_: a class or mapper indicating that criterion will be
against this specific subclass.'

def of_type(self, class_):

    return self.operate(PropComparator.of_type_op, class_)
