private void addParticipant(Source source) {
        assert (validMember(source));
        assert (!source.equals(leader));

        if (!participants.add(source))
            throw new IllegalArgumentException("Source '" + source + "' added twice to the same group");
    }
A set of sources with the same name, each associated with a different provider, that fills the same role.

@author Tony Vaagenes
config-model/src/main/java/com/yahoo/vespa/model/container/search/searchchain/SourceGroup.java