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


DEBUG:Going to execute:
subst Y
Level 1: Backtrace:

Ltac variable X is bound to S n of type constr which cannot be coerced to
a variable.