data Term =
    Nat Integer
    | Abs String Term   -- EAbs Abstraction
    | Inv Term Term     -- EApp Invocation (a.k.a. Application) of a function
    | Var String        -- EVar a variable
    | Add

pretty x = case x of
    (Abs name term) -> "L" ++ name ++ "." ++ pretty term
    (Inv func param) -> "(" ++ pretty func ++ " " ++ pretty param ++ ")"
    (Var name) -> name
    Add -> "Add"

-- example
-- >>> pretty (Abs "x" $ Abs "y" $ Inv (Inv Add (Var "x")) (Var "y"))
-- "Lx.Ly.((Add x) y)"