anchor.diff:  ===== Virtual fields
   
   In addition to the normal fields of the tuples, some tuples have 'virtual'
  -fields, that is fields which value is computed rather than stored. to
  +fields, that is fields which value is computed rather than stored. To
   distinguish them from normal fields their name starts with a dash ('#'). Here
   is a list of all available virtual fields and which tuple they apply to:
   
  
positive.code: previous values instead of combining several current values.
  
  For instance, `max response_time` will compute the max of all the response_time
  fields of all incoming tuples (until the commit-clause instruct Rigatoni to
  output this aggregated tuple).
  
  ===== Min, Max, Sum
  
negative.code: | `out.#successive`
  | In a +GROUP BY+ operations, how many successive incoming tuples were assigned to that group (same `group by` key).
  |===
  
  NOTE: `out.#successive` is unchanged by an aggregate flush operation and
  therefore make little sense in a `remove`/`keep` clause.
  
  ==== Operators and Functions
  
  