Ambiguity of NIL Token

This is regarding the following SPARQL Grammar rules and query (from the 
DAWG test data/SyntaxFull/syntax-lists-05.rq):

[29] ArgList ::= ( NIL | '(' Expression (',' Expression )* ')' )
[39] Collection := '(' GraphNode+ ')'
[56] BrackettedExpression ::= '(' Expression ')'
[85] NIL ::= '(' WS* ')'

SELECT * WHERE { ( ( ) ) }

It's not clear how to interpret the NIL token in the above query.  It's 
not an empty collection or an empty bracketed expression since both of 
these 'must' have content in between the parentheses.

Chimezie Ogbuji
Lead Systems Analyst
Thoracic and Cardiovascular Surgery
Cleveland Clinic Foundation
9500 Euclid Avenue/ W26
Cleveland, Ohio 44195
Office: (216)444-8593
ogbujic@ccf.org