code: private List<String> getUsersFor(Collection<String> roles){
  		List<String> users = new HashList<>();
  		for(String role: roles){
  			if(roleMembershipRU.containsKey(role))
  				users.addAll(roleMembershipRU.get(role));
  		}
  		return users;
  	}
docs: Returns all users with the given role.<br>
  It is assumed that the caller ensures parameter validity.
  @param roles