def init(self, expr):
     super(Except, self).init(expr)
     self.c = Pattern.create(expr.leaves[0])
     if len(expr.leaves) == 2:
         self.p = Pattern.create(expr.leaves[1])
     else:
         self.p = Pattern.create(Expression('Blank'))

def match(expr, form, evaluation):        
    class StopGenerator_MatchQ(StopGenerator):
        pass
    
    form = Pattern.create(form)
    def yield_func(vars, rest):
        raise StopGenerator_MatchQ(Symbol("True"))
    try:
        form.match(yield_func, expr, {}, evaluation)
    except StopGenerator_MatchQ, exc:
        return True

 def apply(self, expr, form, evaluation):
     'MatchQ[expr_, form_]'
     
     class StopGenerator_MatchQ(StopGenerator):
         pass
     
     form = Pattern.create(form)
     #for vars, rest in form.match(expr, {}, evaluation):
     def yield_func(vars, rest):
         #return Symbol("True")
         raise StopGenerator_MatchQ(Symbol("True"))
     try:
         form.match(yield_func, expr, {}, evaluation)
     except StopGenerator_MatchQ, exc:
         return exc.value

 def __init__(self, form):
     self.form = Pattern.create(form)

 def __init__(self, pattern, system=False):
     super(BaseRule, self).__init__()
     self.pattern = Pattern.create(pattern)
     self.system = system

 def __init__(self, pattern, system=False):
     self.pattern = Pattern.create(pattern)
     self.system = system