f true true x s y f isNat x isNat y s x double s y isNat 0 true isNat s x isNat x double 0 0 double s x s s double x 2.2 isNat s x isNat x f true true x s y double s y f true true x s y isNat y f true true x s y f isNat x isNat y s x double s y double s x double x f true true x s y isNat x true f true true x s y f isNat x isNat y s x double s y true Failed! NaTT certifiable-1.6