fold.gl

define foldl(f, e, [])   = e
     | foldl(f, e, x:xs) = foldl(f, f(e, x), xs)

define foldr(f, e, [])   = e
     | foldr(f, e, x:xs) = f(x, foldr(f, e, xs))

define map(f, xs) = foldr(lambda(x, fxs) (f(x):fxs), [], xs)

define op :(x, xs) = if true then [x] else xs