Goal:
  
  A : Type
  a : A
  l : list A
  n : nat
  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))
        (BinInt.Z.add (BinInt.Z.of_nat n) (BinNums.Zpos BinNums.xH))
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat n)
  cstr0 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|l|)
  ============================
   (BinInt.Z.le (BinInt.Z.of_nat #|l|) (BinInt.Z.of_nat n))


DEBUG:Going to execute:
idtac