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


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

Ltac variable X is bound to hd :: tl of type constr which cannot be coerced
to a variable.