wow, thanks for the explanation! I was not aware of these subtle differences between the forth and posctript styles. Having implemented a few stack-based mini-languages for mathematical expressions, I always pick a "greedy" evaluation style, where "ifelse" is a function of three arguments (pops the top three values of the stack) which have been evaluated beforehand. Since there are no side effects, the end result is the same, but it is certainly not a "true" ifelse, since both sides get evaluated regardless of the condition.