public default <E2 extends Number> GraphTraversal<S, E2> meanAgeOfFriends() {
        return ((SocialTraversalDsl) out("knows")).person().values("age").mean();
    }
@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-annotations/src/test/resources/org/apache/tinkerpop/gremlin/process/traversal/dsl/SocialTraversalDsl.java