anchor.diff:  [abstract]
   .Abstract
   --
  -This CIP introduces EXISTS, a general function that can test for the existence of properties, simple patterns and full subqueries.
  +This CIP introduces `EXISTS`, a function and a keyword for checking the existence of properties, simple patterns and full subqueries.
   --
   
   toc::[]
  
positive.code: ----
  
  === Interaction with existing features
  This replaces the pre-existing +HAS(...)+ function.
  
  == What others do
  
  
negative.code:            | simple subquery exists
              ;
  
  property exists = "exists", "(", expression ")"
                  ;
  
  subquery exists = "EXISTS", "{", read only clause, { read only clause }, "}" ;
  