rename (Lambda x t) = Lambda n (substitute x (Var n) t)
where n = rnm x
rnm v = if (v ++ "r") `isFreeIn` t then rnm (v ++ "r") else v ++ "r"
rename (Lambda x t) = Lambda n (substitute x (Var n) t)
where n = rnm x
rnm v = if (v ++ "r") `isFreeIn` t then rnm (v ++ "r") else v ++ "r"