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: the default tuple when the tuple name is omitted.
  
  You can use the `in` tuple in all clauses but in a +YIELD+ operation or a
  +READ+ operation.  When used in the commit clause of a +GROUP BY+ operation,
  it refers to the last received tuple.
  
  [[output-tuple]]
  ===== Output tuple
  
  The tuple that is going to be output. Its name is `out`.  The only place where
  it can be used is in the commit clause and flush-when clause of a +GROUP BY+
  operation. It thus refers to the tuple that would be output shall the commit
  condition yields `true`.
  
  It is important to understand that the input and output tuples have different
  types (at least in general).
  