module Quox.Thin.Append

import public Quox.Thin.Base
import public Quox.Thin.View
import Data.DPair

%default total

public export
app' : OPE m1 n1 mask1 -> OPE m2 n2 mask2 -> Exists (OPE (m1 + m2) (n1 + n2))
app' Stop             ope2 = Evidence _ ope2
app' (Drop ope1 Refl) ope2 = Evidence _ $ Drop (app' ope1 ope2).snd Refl
app' (Keep ope1 Refl) ope2 = Evidence _ $ Keep (app' ope1 ope2).snd Refl

public export
(++) : {n1, n2, mask1, mask2 : Nat} ->
       (0 ope1 : OPE m1 n1 mask1) -> (0 ope2 : OPE m2 n2 mask2) ->
       Subset Nat (OPE (m1 + m2) (n1 + n2))
ope1 ++ ope2 with %syntactic (view ope1)
  Stop           ++ ope2 | StopV = Element _ ope2
  Drop ope1 Refl ++ ope2 | DropV mask ope1 =
    Element _ $ Drop (ope1 ++ ope2).snd Refl
  Keep ope1 Refl ++ ope2 | KeepV mask ope1 =
    Element _ $ Keep (ope1 ++ ope2).snd Refl

-- [todo] this mask is just (mask1 << n2) | mask2
-- prove it and add %transform