code: function addSelector(prefix, selector) {
          if (selector !== undefined) {
              selectors[prefix] = selector;
          }
          return selectors[prefix];
      }
docs: /*
  Add a selector that matches source keys that start with the
  specified prefix.
  
  @param {string} prefix
  @param {function} [selector] The selector for the specified prefix.
  @return {function} The selector for the specified prefix.