The variable assigned in a BIND clause must not already be in-use within the immediately preceding TriplesBlock
within a GroupGraphPattern.
See also https://www.w3.org/TR/sparql12-query/#variableScope
In BIND (expr AS v) requires that the variable v is not in-scope from the preceeding elements in the
group graph pattern in which it is used.
Grammar note 12 of https://www.w3.org/TR/sparql12-query/#sparqlGrammar (note 13 in SPARQL 1.1)