Who Cites sum?
sum	Def sum(f(x) | x < k) == primrec(k;0;x,n. n+f(x))
	Thm* n:, f:(n). sum(f(x) | x < n)
primrec	Def primrec(n;b;c) == if n=0 b else c(n-1,primrec(n-1;b;c)) fi  (recursive)
	Thm* T:Type, n:, b:T, c:(nTT). primrec(n;b;c)  T
eq_int	Def i=j == if i=j true ; false fi
	Thm* i,j:. (i=j)

Syntax:

sum(f(x) | x < k)

has structure:

sum(k; x.f(x))

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