double 0 0 double s x s s double x del x nil nil del x cons y xs if eq x y x y xs if true x y xs xs if false x y xs cons y del x xs eq 0 0 true eq 0 s y false eq s x 0 false eq s x s y eq x y first nil 0 first cons x xs x doublelist nil nil doublelist cons x xs cons double x doublelist del first cons x xs cons x xs 2.2 if false x y xs del x xs doublelist cons x xs first cons x xs del x cons y xs eq x y eq s x s y eq x y doublelist cons x xs del first cons x xs cons x xs doublelist cons x xs double x double s x double x del x cons y xs if eq x y x y xs doublelist cons x xs doublelist del first cons x xs cons x xs true doublelist cons x xs doublelist del first cons x xs cons x xs true Failed! NaTT certifiable-1.6