anchor.diff:             | property exists
              | subquery exists
              | simple subquery exists
  -            ;
  +           ;
   
   property exists = "exists", "(", expression ")"
                   ;
  
positive.code: 
  == Background
  
  In a way, Cypher already has existential subqueries - pattern predicates allow checking for the existence of subgraphs.
  This form is helpful, but not exhaustive in the types of subqueries that users want to be able to express.
  
  == Proposal
  
  To make this feature more powerful, add a new function named `exists`, and a keyword named the same, which allows for two different predicates:
  
   * Property existence checking
   * Subquery existence checking
  
  === Syntax
  
  
negative.code: ----
  
  === Interaction with existing features
  
  The `EXISTS {}` subquery clause renders obsolete the current pattern predicate syntax.
  This allows the pattern predicates to be deprecated and/or removed in favour of `EXISTS {}`.
  
  == What others do
  
  This is very similar to what SQL does with its `EXISTS` functionality.
  
  This is also very similar in syntax to what SPARQL does with its `EXISTS` functionality; the rules regarding variables are identical, and the inner query also takes a subquery as input.
  
  == Benefits to this proposal
  
  