apply env (LamExpr n e) arg = do
    arg' <- subs e n arg
    eval env arg'
-- ^This is most likely an error.
apply env (LamExpr n e) arg = do
    arg' <- subs e n arg
    eval env arg'
-- ^This is most likely an error.
apply env (LamExpr n e) arg = do
    arg' <- subs e n arg
    eval env arg'
-- ^This is most likely an error.