docs: public Membership create() throws RedmineException {
  		if (getProject() == null) {
  			throw new IllegalArgumentException("Project must be set");
  		}
  		if (getUserId() == null && getRoles().isEmpty()) {
  			throw new IllegalArgumentException("Either User or Roles field must be set");
  		}
  		return transport.addChildEntry(Project.class, getProject().getId() + "", this);
  	}