diff --git a/sys/include/mp.h b/sys/include/mp.h
index b49cb16a7..0de6db39c 100644
--- a/sys/include/mp.h
+++ b/sys/include/mp.h
@@ -102,6 +102,9 @@ int	mpcmp(mpint *b1, mpint *b2);
 /* res = s != 0 ? b1 : b2 */
 void	mpsel(int s, mpint *b1, mpint *b2, mpint *res);
 
+/* return n! */
+mpint*	mpfactorial(ulong n);
+
 /* extended gcd return d, x, and y, s.t. d = gcd(a,b) and ax+by = d */
 void	mpextendedgcd(mpint *a, mpint *b, mpint *d, mpint *x, mpint *y);