Goal:
  
  A : Type
  a : A
  l : list A
  IHl : ∀ n : nat, #|l| ≤ n → firstn n l = l
  H : BinInt.Z.le
        (BinInt.Z.add (BinInt.Z.of_nat #|l|) (BinNums.Zpos BinNums.xH))
        BinNums.Z0
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|l|)
  ============================
   ([] = a :: l)


DEBUG:Going to execute:
idtac