Heh, it's a preprocessor DSL:

    #define on(_, x)  if (e.type == _) { x; }
    #define map(k, x) if (e.xkey.keycode == stk(k)) { x; }
    #define grab(...) const char *l[] = { __VA_ARGS__, 0 }; \
                        for (int i = 0; l[i]; i++) XGrabKey(d, stk(l[i]), Mod4Mask, r, 1, 1, 1);