anchor.diff:  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:
   
  -```
  +[source,java]
  +----
   // change
   traversal.getSideEffects().get("m").get()
   // to
   traversal.getSideEffects().get("m")
  -```
  +----
   
   TraversalSideEffects Registration Requirement
   +++++++++++++++++++++++++++++++++++++++++++++
  
positive.code: 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()`.
  
  ```
  gremlin> traversal = g.withSideEffect('a',0,sum).V().out().sideEffect{it.sideEffects('a',1)}
  ==>v[3]
  ==>v[2]
  
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]
  