rem: assert("bad_root_window" && 0);  return NULL;
add: return wScreenForWindow(window);
context: wScreenForRootWindow(Window window){    int i;    if (wScreenCount==1)	return wScreen[0];    /*     * Since the number of heads will probably be small (normally 2),     * it should be faster to use this than a hash table, because     * of the overhead.     */    for (i=0; i<wScreenCount; i++) {	if (wScreen[i]->root_win == window) {	    return wScreen[i];	}    }    assert("bad_root_window" && 0);        return NULL;}