* @param s1, s2 - The two systems
* NOTE: In single threaded version, called with s1 == s2 == &universe
* In multi threaded version, called with s2 == &universe, but s1 is constructed for each thread
+ * In nested multi-threaded version, s2 is constructed for the nested threads as well
*/
void System_Forces(System * s1, System * s2)
{