anchor.diff:  must:  PLUS;
   mustNot: MINUS;
   
  -termQuery: TERMSTRING;
  +term: TERMSTRING;
  +fieldName : FIELDNAME;
   
   fragment
  -TermChar :   [:a-zA-Z];
  +TermChar :   [a-zA-Z];
  +
  +fragment
  +FieldNameChar : [a-zA-Z0-9_];
   
   AND 	: 'AND';
   OR  	: 'OR';
  
positive.code: 	| (booleanPrefix? termQuery ')'+)
  	;
  
  booleanOperator: opAnd | opOr;
  booleanPrefix: must | mustNot;
  
  
negative.code: MINUS	: '-';
  
  TERMSTRING : TermChar+ ;
  FIELDNAME : FieldNameChar+ ;
  
  WS : [ \t]+ -> skip;
  
  