anchor.diff:  [source,sql]
   -----
   SELECT word, SUM 1 AS count FROM splitter GROUP BY word
  -----
  +-----
   
   Equivalently, there is also the _special field_ +group.#count+ that counts
   how many elements have been added to a group. We will see later about special
   fields.
   
  -The main difference with SQL, though, is the lack of an implicit moment when
  -to stop aggregating. For such a simple problem as word counting, traditional
  -windowing where we issue an output a flush the aggregation when some
  -condition is true is not powerful enough: we want a new tuple each time a
  -count changes, but we want to count globally. If we did:
  +The main difference with SQL, though, is the lack of an implicit moment when to
  +stop aggregating. For such a simple problem as word counting, traditional
  +windowing where we issue a tuple and flush the aggregation when some condition
  +is met won't cut it: we want a new tuple each time a count changes, but we want
  +to keep forever increasing the counters.
  +
  +If we did:
   
   [source,sql]
   ----
  
positive.code: 
  [NOTE]
  To understand why we would have a count of 2 rather than 1, you must be aware
  that the previous tuple is initially initialized with the first one, to avoid
  having to deal with yet another case of nulls. So when a word is seen for the
  first time it's previous.count is not 0 as you might expect but equal to the
  current value of count, out.count, that is 1. So one must wait until the
  second occurrence of that word for the +COMMIT+ condition to be true.
  
  What we really want to do is to aggregate the counts forever, but still emit
  a new tuple at every change (aka at every step). Fortunately, we can set
  a different condition for when to +COMMIT+ a result (ie. output the result
  tuple) and when to +FLUSH+ the aggregated group:
  
  [source,sql]
  ----
  
negative.code: ----
  
  Simple, and does the work. You will see later that, not only can we set a
  specific condition as to when to flush but we can also select which tuples to
  flush and which to keep from one window to the next.
  
  There you have it. We added an +EXPORT+ keyword at the end of this new
  