code: func (c *Comm) serverCertsInUse() StringSet {
  	endpointsInUse := make(StringSet)
  	for _, mapping := range c.Chan2Members {
  		endpointsInUse.union(mapping.ServerCertificates())
  	}
  	return endpointsInUse
  }
docs: // serverCertsInUse returns the server certificates that are in use
  // represented as strings.