There are two more, significantly less SPL-intense options: Cheat. Given the restrictions of finite-bit maths, you can easily ship a lookup of all reasonable factorials instead of calculating them on the fly, here's a start: https://oeis.org/A000142/list Use Stirling's approximation, expressed in SPL it's `| eval n! = sqrt(2*pi()*n)*pow(n/exp(1), n)` ... it won't be perfect, but it'll give you the right number of digits and the first two will be accurate, ie two sigfigs of precision. Probably good enough for your use case, for 13 choose 6 it yields 1749.
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