USP
tp->tv_sec += tp->tv_usec / USP;
tp->tv_usec %= USP;
assert (a->tv_usec <= USP);
assert (b->tv_usec <= USP);
a->tv_sec += b->tv_sec + a->tv_usec / USP;
a->tv_usec %= USP;
assert (a->tv_usec <= USP);
assert (b.tv_usec <= USP);
a->tv_usec += USP;