anchor.diff:  Sack Step
   ~~~~~~~~~
   
  -A traverser can contain a traverser-local sideEffect data structure called a "sack". The `sack()`-step is used to manipulate traverser sacks (*sideEffect* or *map*). Each sack of each traverser is created when the traverser is generated using `withSack(initialValueSupplier,splitOperator?,mergerOperator?)`.
  +A traverser can contain a traverser-local sideEffect data structure called a "sack". The `sack()`-step is used to manipulate traverser sacks (*sideEffect* or *map*). Each sack of each traverser is created when the traverser is generated using `withSack(initialValueSupplier,splitOperator?,mergeOperator?)`.
   
   * *Initial value supplier*: A `Supplier` providing the initial value of each traverser's sack.
   * *Split operator*: a `UnaryOperator` that clones the traversers sack when the traverser splits. If no split operator is provided, then it assumes `UnaryOperator.identity()`.
  
positive.code: ==>tinkergraph[vertices:6 edges:6]
  gremlin> sg = TinkerGraph.open() <1>
  ==>tinkergraph[vertices:0 edges:0]
  gremlin> g.E().with('sg',sg).subgraph('sg', {it.label == 'knows'}) <2>
  ==>tinkergraph[vertices:3 edges:2]
  gremlin> sg.E() <3>
  ==>e[7][1-knows->2]
  
negative.code: ==>v[1]
  ==>v[4]
  
  [[traversalstrategy]]
  TraversalStrategy
  -----------------
  