Ltac jump_simpl :=
  repeat
    match goal with
      | |- context [jump xH] => rewrite (jump_simpl xH)
      | |- context [jump (xO ?p)] => rewrite (jump_simpl (xO p))
      | |- context [jump (xI ?p)] => rewrite (jump_simpl (xI p))
    end.

Lemma jump_tl j l x : tail (jump j l) x = jump j (tail l) x.