code: function parseStructureFile(book, type) {
      var fs = book.getContentFS();
  
      return lookupStructureFile(book, type)
      .then(function(file) {
          if (!file) return [undefined, undefined];
  
          return parseFile(fs, file, type);
      });
  }
docs: Parse a structure file (ex: SUMMARY.md, GLOSSARY.md).
  It uses the configuration to find the specified file.
  
  @param {Book} book
  @param {String} type: one of ["glossary", "readme", "summary"]
  @return {Promise<List|Map>}