anchor.diff:  [source,sql]
   ----
     SELECT avg salary, *
  -  FROM employee GROUP BY age, is_make
  +  FROM employee GROUP BY age, is_male
   ----
   
   ...would output tuples made of the average value of the input field +salary+
  
positive.code: WHERE condition
  FROM source
  GROUP BY expression1, expression2, ...
  TOP number BY expression
  COMMIT [ AND [ FLUSH | SLIDE n | REMOVE condition | KEEP condition ] ] WHEN condition
  ( FLUSH | SLIDE n | REMOVE condition | KEEP condition ) WHEN condition
  EXPORT [ EVENT STARTING AT identifier [ * scale ]
               [ WITH DURATION [ identifier [ * scale ] | constant ] |
  
negative.code: ----
  
  Each of those clauses can be specified in any order and can be omitted but for
  the +FROM+ clause; Also, when +TOP+ is specified then no +COMMIT+ or +FLUSH+
  must be present.
  
  The semantic is:
  
  