English
For p, q and i, smashing p with q respects natural-number length addition: smash p q h (i.natAdd p.length).castSucc = q i.castSucc.
Русский
Слияние p и q с учётом натурального дополнения длины: smash p q h (i.natAdd p.length).castSucc = q i.castSucc.
LaTeX
$$$ smash\ p\ q\ h (i.natAdd p.length).castSucc = q i.castSucc $$$
Lean4
theorem smash_natAdd {p q : RelSeries r} (h : p.last = q.head) (i : Fin q.length) :
smash p q h (i.natAdd p.length).castSucc = q i.castSucc :=
by
dsimp only [smash, Fin.castSucc_natAdd]
apply Fin.addCases_right