Goal:
  
  A : Type
  sizeA : A → nat
  B : Type
  sizeB : B → nat
  a : A
  l : list A
  f : nat → A → B
  k : nat
  H : ∀ (i : nat) (x : A), sizeB (f i x) = sizeA x
  IHl : ∀ k : nat, list_size sizeB (mapi_rec f l k) = list_size sizeA l
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat (sizeA a))
  cstr0 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat (list_size sizeA l))
  ============================
   (BinInt.Z.add
      (BinInt.Z.add (BinInt.Z.of_nat (sizeA a))
         (BinInt.Z.of_nat (list_size sizeA l))) (BinNums.Zpos BinNums.xH) =
    BinInt.Z.add
      (BinInt.Z.add (BinInt.Z.of_nat (sizeA a))
         (BinInt.Z.of_nat (list_size sizeA l))) (BinNums.Zpos BinNums.xH))


DEBUG:Going to execute:
<coq-core.plugins.zify::TRANS@1>