L1, L2:Top List, n:(||L1||+1). firstn(n;L1 @ L2) ~ firstn(n;L1) 

latex

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