Sort of derived from http://www.loc.gov/standards/sru/cql/

cqlQuery ::= prefixAssignment cqlQuery | scopedClause
prefixAssignment ::= '>' prefix '=' uri | '>' uri
scopedClause ::= scopedClause booleanGroup searchClause | searchClause
booleanGroup ::= boolean [modifierList]
boolean ::= 'and' | 'or' | 'not' | 'prox'
searchClause ::= '(' cqlQuery ')'
	       | index relation searchTerm
	       | searchTerm
relation ::= comparitor [modifierList]
comparitor ::= comparitorSymbol | namedComparitor
comparitorSymbol ::= '=' | '>' | '<' | '>=' | '<=' | '<>'
namedComparitor ::= identifier
modifierList ::= modifierList modifier | modifier
modifier ::= '/' modifierName [comparitorSymbol modifierValue]
prefix, uri, modifierName, modifierValue, searchTerm, index ::= term
term ::= identifier | 'and' | 'or' | 'not' | 'prox'
identifier ::= charString1 | charString2