hmm...maybe WildFunction is what you need here....nope:

```
>>> F=WildFunction('F')
>>> sin(x).match(F(x))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'WildFunction' object is not callable
>>> sin(x).match(F)
{F_: sin(x)}
```
It would be nice if there were a WildAppliedUndef class but I don't know of one. It looks like this is a two step match.