code: func (m *MonotonicEntropy) increment() error {
  	if inc, err := m.random(); err != nil {
  		return err
  	} else if m.entropy.Add(inc) {
  		return ErrMonotonicOverflow
  	}
  	return nil
  }
docs: // increment the previous entropy number with a random number
  // of up to m.inc (inclusive).