let_bind env (b:e:xs) = do
    evaled <- eval e env
    x <- liftIO $ env_set env b evaled
    let_bind env xs
let_bind env (b:e:xs) = do
    evaled <- eval e env
    x <- liftIO $ env_set env b evaled
    let_bind env xs
let_bind env (b:e:xs) = do
    evaled <- eval e env
    x <- liftIO $ env_set env b evaled
    let_bind env xs