code: public StandardRelationTypeMaker sortKey(PropertyKey... keys) {
          Preconditions.checkArgument(keys!=null && keys.length>0);
          sortKey.addAll(Arrays.asList(keys));
          return this;
      }
docs: Configures the composite sort key for this label.
  <p/>
  Specifying the sort key of a type allows relations of this type to be efficiently retrieved in the order of
  the sort key.
  <br />
  For instance, if the edge label <i>friend</i> has the sort key (<i>since</i>), which is a property key
  with a timestamp data type, then one can efficiently retrieve all edges with label <i>friend</i> in a specified
  time interval using {@link com.thinkaurelius.titan.core.TitanVertexQuery#interval(com.thinkaurelius.titan.core.PropertyKey, Comparable, Comparable)}.
  <br />
  In other words, relations are stored on disk in the order of the configured sort key. The sort key is empty
  by default.
  <br />
  If multiple types are specified as sort key, then those are considered as a <i>composite</i> sort key, i.e. taken jointly
  in the given order.
  <p/>
  {@link com.thinkaurelius.titan.core.RelationType}s used in the sort key must be either property out-unique keys or out-unique unidirected edge lables.
  
  @param keys TitanTypes composing the sort key. The order is relevant.
  @return this LabelMaker