anchor.diff:  Thus, the below will increment "a". If no operator was provided, then the operator is assumed `Operator.assign` and the
   final result of "a" would be 1. Note that `Traverser.sideEffects(key,value)` uses `TraversalSideEffect.add()`.
   
  -```
  +[source,groovy]
  +----
   gremlin> traversal = g.withSideEffect('a',0,sum).V().out().sideEffect{it.sideEffects('a',1)}
   ==>v[3]
   ==>v[2]
  
positive.code: directly return the side-effect value to reduce object creation costs and because all side-effects must be registered apriori,
  there is never a reason why an unknown side-effect key would be used. In short:
  
  ```
  // change
  traversal.getSideEffects().get("m").get()
  // to
  traversal.getSideEffects().get("m")
  ```
  
  TraversalSideEffects Registration Requirement
  +++++++++++++++++++++++++++++++++++++++++++++
  
negative.code: ==>v[3]
  gremlin> traversal.getSideEffects().get('a')
  ==>1
  ----
  
  See: link:https://issues.apache.org/jira/browse/TINKERPOP-1192[TINKERPOP-1192],
  https://issues.apache.org/jira/browse/TINKERPOP-1166[TINKERPOP-1166]
  