add a temporary faculty function

src/exponent.c

@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include <limits.h>
 
 #include "exponent.h"
 
@@ -25,3 +26,16 @@ double expIN(double e, double num){
     }
     return 0;
 }
+
+unsigned long long fac(int x){
+    unsigned long long prod = 1;
+    if(x==0) return 1;
+    for (int i = 1; i <= x; i++){
+        if (prod > ULLONG_MAX / i){
+            break;
+        }
+        prod*=i;
+    }
+    return prod;
+}
+