English
Similarly, the i-th element on the right part of p.concat(q) equals the i-th element of q for i in the right segment.
Русский
Аналогично, i-й элемент правой части p ∘ q равен i-му элементу q для i в правой части.
LaTeX
$$$ (p \\append q \\;\\text{connect}).toFun (i) = q(i) \\quad\\text{for } i < q.length + 1 $$$
Lean4
theorem append_apply_right (p q : RelSeries r) (connect : p.last ~[r] q.head) (i : Fin (q.length + 1)) :
p.append q connect ((i.natAdd (p.length + 1)).cast (by dsimp; cutsat) : Fin ((p.append q connect).length + 1)) =
q i :=
Fin.append_right _ _ _