Tail
Recursion
fac(n)
= f1(1, n, 1);
fac(n) = n <= 1 ? 1 : n*fac(n-1);f1(a, n, x) = x <= n ? f1(a*x, n, x+1)
:
a;
f1(a*x, n, x+1) :
a;tail-recursive
non-tail-recursive
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However,
tail-recursive functions may be harder to read.