Look, a model checker! More seriously, it's interesting how this kind of thing is trivial to express in some languages (TLA+, prolog, Alloy), ok in some (Ruby,

Look, a model checker! More seriously, it's interesting how this kind of thing is trivial to express in some languages (TLA+, prolog, Alloy), ok in some (Ruby, scheme), and super hard in others. The ruby model and specification would translate almost line-by-line into TLA+, and TLC could do what the backtracking implementation does.