T:Type, a:T, as:(T List), i:. (i  0)  (cons(a; as)[i] = a) 

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Definitions	t  T, l[i], P  Q, x:A. B(x), T, ff, P  Q, P  Q, P  Q, tt, if b then t else f fi , Y, nth_tl(n;as), True, prop{i:l}, hd(l), Unit, , False, A, A  B,
Lemmas	le wf, assert of lt int, bnot of le int, true wf, squash wf, eqff to assert, assert of le int, eqtt to assert, iff transitivity, bnot wf, lt int wf, assert wf, bool wf, le int wf