anchor.diff:  ----
   <Atom> ::=
       ...
  -  | <RelationshipsPattern>
  +  | <PatternPredicate>
     | <ExistentialSubquery>
  +
  +<PatternPredicate> ::=
  +  <RelationshipsPattern>
   
   <ExistentialSubquery> ::=
       <SimpleExistentialSubquery>
  
positive.code: * Are side effect-free, i.e. <FullExistentialSubquery> shall not contain any <UpdatingClause> or other sources of side effects
  * Can contain variables and parameters from the outer queries
  
  https://raw.githack.com/openCypher/openCypher/master/tools/grammar-production-links/grammarLink.html?p=RelationshipsPattern[<RelationshipsPattern>] shall not introduce new variables.
  
  Both forms of https://raw.githack.com/openCypher/openCypher/master/tools/grammar-production-links/grammarLink.html?p=ExistentialSubquery[<ExistentialSubquery>] (<SimpleExistentialSubquery> and <FullExistentialSubquery>) are allowed to introduce new variables.
  These variables necessarily shall have a name different from the names of all variables available from the outer queries.
  Any variables introduced in an <ExistentialSubquery> are not available outside the subquery context.
  
  https://raw.githack.com/openCypher/openCypher/master/tools/grammar-production-links/grammarLink.html?p=RelationshipsPattern[<RelationshipsPattern>] and <SimpleExistentialSubquery> are syntactically simpler and semantically less powerful forms of <FullExistentialSubquery>.
  The semantics of https://raw.githack.com/openCypher/openCypher/master/tools/grammar-production-links/grammarLink.html?p=RelationshipsPattern[<RelationshipsPattern>] and <SimpleExistentialSubquery> can be defined as syntax transformations to <FullExistentialSubquery>.
  
  ==== <RelationshipsPattern>
  
  A https://raw.githack.com/openCypher/openCypher/master/tools/grammar-production-links/grammarLink.html?p=RelationshipsPattern[<RelationshipsPattern>] _RP_ is effectively replaced by the expression
  
  `EXISTS { MATCH _RP_ RETURN 1 }`
  
  ==== <SimpleExistentialSubquery>
  
  
negative.code: 
  This CIP proposes:
  
  * Refer to a https://raw.githack.com/openCypher/openCypher/master/tools/grammar-production-links/grammarLink.html?p=RelationshipsPattern[<RelationshipsPattern>] appearing as an https://raw.githack.com/openCypher/openCypher/master/tools/grammar-production-links/grammarLink.html?p=Atom[<Atom>] as a https://raw.githack.com/openCypher/openCypher/master/tools/grammar-production-links/grammarLink.html?p=PatternPredicate[<PatternPredicate>] for better distinguishing it from a https://raw.githack.com/openCypher/openCypher/master/tools/grammar-production-links/grammarLink.html?p=RelationshipsPattern[<RelationshipsPattern>] appearing in a https://raw.githack.com/openCypher/openCypher/master/tools/grammar-production-links/grammarLink.html?p=PatternComprehension[<PatternComprehension>]
  * Restricting https://raw.githack.com/openCypher/openCypher/master/tools/grammar-production-links/grammarLink.html?p=PatternPredicate[<PatternPredicate>] to predicate contexts and boolean evaluation, when used in expression contexts
  * Adding two forms of existential subqueries denoted with `EXISTS { ... }`:
  ** Full existential subquery based on a https://raw.githack.com/openCypher/openCypher/master/tools/grammar-production-links/grammarLink.html?p=RegularQuery[<RegularQuery>]
  ** Simple existential subquery based on a single https://raw.githack.com/openCypher/openCypher/master/tools/grammar-production-links/grammarLink.html?p=Pattern[<Pattern>] and an optional https://raw.githack.com/openCypher/openCypher/master/tools/grammar-production-links/grammarLink.html?p=Where[<Where>] clause
  