docs: public ImmutableListMultimap<Symbol, EDLMention> loadEDLMentionsByDocFrom(CharSource source) throws IOException {
      final ImmutableList<EDLMention> edlMentions = loadEDLMentionsFrom(source);
      final ImmutableListMultimap.Builder<Symbol, EDLMention> byDocs =
          ImmutableListMultimap.<Symbol, EDLMention>builder()
              .orderKeysBy(SymbolUtils.byStringOrdering());
      for (final EDLMention edlMention : edlMentions) {
        byDocs.put(edlMention.documentID(), edlMention);
      }
      return byDocs.build();
    }