code: func (lr *LockedRand) Int63n(n int64) (r int64) {
  	lr.lk.Lock()
  	r = lr.r.Int63n(n)
  	lr.lk.Unlock()
  	return
  }
docs: // Int63n returns, as an int64, a non-negative pseudo-random number in [0,n).
  // It panics if n <= 0.