code: func (c *Config) mutualVersion(vers uint16) (uint16, bool) {
  	minVersion := c.minVersion()
  	maxVersion := c.maxVersion()
  
  	if vers < minVersion {
  		return 0, false
  	}
  	if vers > maxVersion {
  		vers = maxVersion
  	}
  	return vers, true
  }
docs: // mutualVersion returns the protocol version to use given the advertised
  // version of the peer.