Intuitively you only use the value of X in that case, not the derivative. I don't know if that yields a consistent or useful language, though, since you'd also

Intuitively you only use the value of X in that case, not the derivative. I don't know if that yields a consistent or useful language, though, since you'd also need to consider the reverse operation. In general I think that languages that need such a calculus (Modelica's functions come to mind) would show a lot of such complications when implements a rigorous description of the semantics.