Goal:
  
  A : Type
  l : list A
  n0 : {n : nat | n < #|l|}
  hd : A
  tl : list A
  Heq_l : hd :: tl = l
  n : nat
  Heq_n : S n = proj1_sig n0
  ============================
   (n < #|tl|)


DEBUG:Going to execute:
unfold Tactics.fix_proto in *
Level 1: Backtrace:

No matching clauses for match.
Level 0: Backtrace:

No matching clauses for match.