Goal:
    A : Type  l : list A  x : A
  ============================
   (l = l)


DEBUG:Going to execute:
done
DEBUG LTAC Evaluated term: firstn_app
DEBUG LTAC Evaluated term: firstn_app
DEBUG LTAC Evaluated term: Nat.sub_diag
DEBUG LTAC Evaluated term: Nat.sub_diag
DEBUG LTAC Evaluated term: Prop
DEBUG LTAC Evaluated term: Prop
DEBUG LTAC Evaluated term: firstn_all
DEBUG LTAC Evaluated term: firstn_all
DEBUG LTAC Evaluated term: app_nil_r
DEBUG LTAC Evaluated term: app_nil_r
DEBUG LTAC Evaluated term: Prop
DEBUG LTAC Evaluated term: Prop