code: public static ByteSource concatOpenable(Iterable<ByteSource> possibleBrokenSources) {
          //noinspection StaticPseudoFunctionalStyleMethod
          return ByteSource.concat(Iterables.transform(possibleBrokenSources, ByteSources::orEmpty));
      }
docs: Concatenates multiple byte sources, skipping sources for which
  opening an input stream fails.
  @param possibleBrokenSources an iterable over the possibly-broken sources
  @return the concatenated source