Error from SPARQL Exists Community Group: Substitution affects disconnected variables

[pfps]

The SPARQL Exists Community Group, in https://w3c.github.io/sparql-exists/docs/sparql-exists.html,
has described errors in the SPARQL Query specification having to do with substitution affecting disconnected variables in https://w3c.github.io/sparql-exists/docs/sparql-exists.html#issue-5-substitution-affects-disconnected-variables

For example, in

  SELECT ?x WHERE {
    BIND ( :d AS ?x )
    FILTER EXISTS { BIND ( :e AS ?z )
                    SELECT ?y WHERE { ?x :p :c } } }

the substitution from 18.6 ends up producing

  Join ( Extend( BGP(), ?z :e ),
         ToMultiSet( Project( ToList( BGP( :d :p :c ) ), { ?y } ) ) )

Some, but not all, implementations diverge from the spec here.

These errors should be fixed.

Issue #5 is an overall tracker for issues from the SPARQL Exists Community Group.