anchor.diff:  
   attribute : IDENTIFIER ':' oclType '=' oclExpression;
   
  -rule : calledRule | matchedRule;
  +arule : calledRule | matchedRule;
   
   matchedRule : lazyMatchedRule | matchedRule_abstractContents;
   
  
positive.code: 
  libraryRef : 'uses' STRING ';';
  
  moduleElement : helper | rule;
  
  helper : 'helper' oclFeatureDefinition ';';
  
  
negative.code: 
  LINE_COMMENT
  	:   '--' ~[\r\n]* -> skip
  	;
  