Goal:
    A : Type  l : list A  n : nat  H : n < #|l|  Heq_l : [] = l
  ============================
   A


DEBUG:Going to execute:
Tactics.destruct_one_pair || Tactics.destruct_one_ex
Level 7: evaluation returns
destruct H as [H ph]
where
ph := H
of type
intropattern
H := n
of type
constr of type tacvalue