n:, f:(n), a:. asum(f(x) | x < n) = sum(af(x) | x < n) 

latex

Definitions	ij, sum(f(x) | x < k), S  T, S  T, Unit, P  Q, i=j, , b, Prop, b, , {i..j}, i  j < k, P & Q, AB, A, False, P  Q, primrec(n;b;c), x:A. B(x), t  T, x(s)
Lemmas	le wf, primrec wf, assert wf, not wf, bnot wf, bool wf, eq int wf, assert of eq int, not functionality wrt iff, assert of bnot, iff transitivity, eqff to assert, eqtt to assert, int seg wf, nat wf, nat properties, ge wf