rasqal_expression variable binding ambiguity

gistfile1.sql

; Current API cannot distinguish these since (variable, binding expr) are in same
; structure identified globally by variable name.

1. 

SELECT ((1+1) AS ?foo)
WHERE { ... }
GROUP BY ?foo

2.
SELECT ?foo
WHERE { ... }
GROUP BY ((1+1) AS ?foo)


3.
SELECT ?foo
WHERE { ...
  BIND ((1+1) AS ?foo)
... }
GROUP BY ?foo