Goal:
  
  A : Type
  n : nat
  a : A
  l : list A
  IHl : ∀ n : nat, #|firstn n l| = n → n ≤ #|l|
  ============================
   (S #|firstn n l| = S n → S n ≤ S #|l|)


DEBUG:Going to execute:
<coq-core.plugins.ltac::auto@0> $1 $2 $3