Summary: Fakery with _plus can only go so far -- we need a _minus as well.
Original report, from Eric:
I want to subtract a non-numeric-constant value from another, which I can't seem to contort _plus to manage. (_plus(-1) is fine, but _plus(-$commentLimit) errors.)
There's no reason not to just do this: it's very easy to implement, along the model of _plus.
