anchor.diff:  the output type of the parent nodes. Therefore changing an ancestor
   operation might change the output type of an unmodified node.
   
  -The output tuple is then send to each of the children nodes, before a new input
  +The output tuple is then sent to each of the children nodes, before a new input
   tuple is read. No batching takes place in the nodes, although batching does
   take place in the communication in between them (the ring-buffers).  Indeed,
   when a node has no tuple to read it _sleeps_ for a dynamic duration that is
  
positive.code: GROUP BY expression1, expression2, ...
  ----
  
  You can mix stateful functions drawing their state from the group the tuple at
  hands belong to, with stateful functions having a global state. Where a
  stateful function draws its state from depends on the presence or absence of
  the `globally` modifier to the function name. For instance, let's also comp[ute
  the global average salary:
  
  [source,sql]
  ----
  
negative.code: seasonality) and `k` (how many seasons in the past to consider). Notice that
  in the example above as well as in the functions below the current value is
  skipped: `v(t)` is not in the average. This is because we often want to
  compare the past seasons with the current value.
  
  Seasonality is similar to fixed length windows but implemented at the
  function level rather than at the aggregation level.
  