code: func (ung UniformGenerator) Int64n(n int64) int64 {
  	if n <= 0 {
  		panic(fmt.Sprintf("Illegal parameter n: %d", n))
  	}
  	ung.mu.Lock()
  	defer ung.mu.Unlock()
  	return ung.rd.Int63n(n)
  }
docs: // Int64n returns a random uint64 in [0, n)