T:Type, as, bs:T List, i:(||as||+||bs||).
(as @ bs)[i] = if i<||as|| as[i] else bs[i-||as||] fi  T 

latex

Definitions	i  j < k, ||as||, {i..j}, t  T, x:A. B(x), Prop, True, ij, b, b, , AB, i<j, P  Q, T, P  Q, P & Q, P  Q, Unit
Lemmas	int seg wf, eqtt to assert, assert of lt int, iff transitivity, eqff to assert, squash wf, true wf, bnot of lt int, assert of le int, lt int wf, le wf, bool wf, bnot wf, assert wf, le int wf, length wf1, select append front, select append back