'Return a list of all useful sources. Where there are multiple sources
associated with the same frag_index then only one is included.
:return: a list of sources, each source being a tuple of form
(ResumingGetter, iter)'

def get_responses(self):

    all_sources = []
    for (frag_index, sources) in self.gets.items():
        if (frag_index is None):
            all_sources.extend(sources)
        else:
            all_sources.extend(sources[:1])
    return all_sources
