anchor.diff:  |`FLOAT`        |        |                |        |       |                |          |           | X         | X       |
   |`VOID`         |        |                |        |       |                |          |           |           |         |
   |================================================================================================================================
  -`
  +
  
positive.code: In a regular aggregation (i.e. of the form `aggr(expr)`), the list of aggregated values is the list of candidate values with all `null` values removed from it.
  
  In a distinct aggregation (i.e. of the form `aggr(DISTINCT expr)`), the list of aggregated values is the list of candidate values with all `null` values removed from it.
  Futhermore, in a distinct aggregation, only one of all equivalent candidate values is included in the list of aggregated values, i.e. duplicates under equivalence are removed.
  However, if the distinct aggregation happens in a projection with an associated `ORDER BY` subclause, always the first from a set of equivalent candidate values is included in the list of aggregated values.
  
  Finally, the remaining aggregated values are processed by the actual aggregation function.
  If the list of aggregated values is empty, the aggregation function returns a default value (`null` unless specified otherwise below).
  
negative.code: :toc-placement: macro
  :source-highlighter: codemirror
  
  *Authors:* Mats Rydberg <mats@neotechnology.com>, Stefan Plantikow <stefan.plantikow@neotechnology.com>
  
  [abstract]
  .Abstract
  