| contains(str, ty0, ty1), contains(str, ty, ty0) ⊦ contains(str, ty, ty1) |
⊦ contains(str, ty, ty0) ↔ (∃ tys. contains(str, ty ' + (tys + ty0)) ∧ ¬ dups(ty ' + tys))
∃ tys. contains(str, ty0 ' + (tys + ty1)) ∧ ¬ dups(ty0 ' + tys), contains(str, ty, ty0) ⊦ contains(str, ty, ty1)
[[CONCRETESIMPRULES : ()], [EXTRATERMS : (ty1, ty0, str)]]