L1,L2:(top List), n:int_seg(0; (||L1|| + 1)).
sqequal(firstn(n; append(L1; L2)); firstn(n; L1)) 

latex

Definitions	x:A. B(x), ||as||, firstn(n; as), append(as; bs), Y, t  T, if b then t else f fi , P  Q, tt, P  Q, prop{i:l}, P  Q, P  Q, ff, T, True, int_seg(i; j), , Unit,
Lemmas	top wf, lt int wf, bool wf, iff transitivity, assert wf, eqtt to assert, assert of lt int, le int wf, le wf, bnot wf, eqff to assert, squash wf, true wf, bnot of lt int, assert of le int, int seg wf, length wf1