anchor.diff:  MANDATORY MATCH (p:Product {id: {productId}})
   RETURN (u)-[:OWNS]->(p)
   ----
  +
  +In a very common variant of this scenario, nodes will be selected based on a uniqueness constraint.
   
   === Syntax
   
  
positive.code: 
  == Benefits to this proposal
  
  Application programmers get a new powerful facility for detecting semantical errors in their applications while
  also failing early in the case of an error (thus avoiding unnecessary work).
  
  == Caveats to this proposal
  
  
negative.code: 
  === Alternatives
  
  Care should be taken to not confuse use-cases: This CIP allows applications to propagate some of their argument verification to Cypher by providing a fail-early mechanism for non-matching patterns.
  
  Alternatively, Cypher could provide a more general, heavyweight mechanism for raising errors.
  Such a facility might also achieve the goals of this CIP while additionally supporting other use-cases.
  However in the interest of keeping this proposal small and specific, this is not considered here.
  
  == What others do
  