code: function _gpfReduceContext (path, reducer) {
      var rootContext,
          pathToReduce;
      if (path[_GPF_START] === "gpf") {
          rootContext = gpf;
          pathToReduce = _gpfArrayTail(path);
      } else {
          rootContext = _gpfMainContext;
          pathToReduce = path;
      }
      return pathToReduce.reduce(reducer, rootContext);
  }
docs: Apply reducer on path