Simplifying Using Substitution
fac(n)
= f1(1, n, 1);
f1(a, n, x) = x <= n ? 
f2(a, n, x)
: a;
f2(a, n, x) : a;f2(a, n, x) = f1(a*x, n, x+1);
f1(a, n, x) = x <= n ? f1(a*x, n, x+1)
:
a;
f1(a*x, n, x+1) :
a;