interp (App t1 t2) env = do
  f <- interp t1 env
  v <- interp t2 env
  apply f v
interp (App t1 t2) env = do
  f <- interp t1 env
  v <- interp t2 env
  apply f v
interp (App t1 t2) env = do
  f <- interp t1 env
  v <- interp t2 env
  apply f v