code: function _parseVertex(line) {
          var vertex = JSON.parse(line);
          assert.ok(_smellsLikeAnElement(vertex));
          // A vertex is an object, i.e. a key,value map.
          // We don't sort the keys of the object, leaving that to jsonStableStringify below.
          // But a vertex values contain `property containers`, each of which may contain embedded arrays of elements.
          return _.mapValues(vertex, _sortPropertyContainers);
      }
docs: ### `_parseVertex()` Parse one line of text from a Tinkerpop graphson stream, yielding one vertex.