Goal:
  
  A : Type
  a : A
  l1 : list A
  l2 : list A
  IHl1 : ∀ l2 : list A, skipn #|l1| (l1 ++ l2) = l2
  ============================
   (skipn #|l1| (l1 ++ l2) = l2)


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