MAYBE

Problem:
 f(true(),xs) -> f(isList(xs),append(cons(a(),nil()),xs))
 isList(nil()) -> true()
 isList(cons(x,xs)) -> isList(xs)
 append(nil(),ys) -> ys
 append(cons(x,xs),ys) -> cons(x,append(xs,ys))

Proof:
 Open