Goal:
  
  A : Type
  B : Type
  f : nat → B
  a : A
  l : list A
  IHl : ∀ f : nat → B,
          mapi_rec (λ (i : nat) (_ : A), f i) l 0 = unfold #|l| f
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|l|)
  ============================
   (BinInt.Z.add (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH) =
    BinInt.Z.add (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH))


DEBUG:Going to execute:
idtac