True -> do { (env'', arg') <- makeTrivial env' arg
; return (True, env'', App fun' arg') }
False -> return (False, env, App fun arg) }
go n_val_args env (Var fun)
True -> do { (env'', arg') <- makeTrivial env' arg
; return (True, env'', App fun' arg') }
False -> return (False, env, App fun arg) }
go n_val_args env (Var fun)