 recursive) function  uniformly across unchecked cases
appdown :: (Term -> Term) -> Term -> Term
appdown f (Lambda v s t) = (Lambda v s (f t))
appdown f (Apply a 
appdown :: (Term -> Term) -> Term -> Term
appdown f (Lambda v s t) = (Lambda v s (f t))
appdown f (Lambda v s t) = (Lambda v s (f t))