def each_source_info source_id, prefix='', &b
    p = mkterm :location, source_id, prefix
    each_prefixed_term p do |x|
      yield prefix + x[p.length..-1]
    end
  end
def each_source_info source_id, prefix='', &b
    p = mkterm :location, source_id, prefix
    each_prefixed_term p do |x|
      yield prefix + x[p.length..-1]
    end
  end
Yields (in lexicographical order) the source infos of all locations from
 the given source with the given source_info prefix