anchor.diff:      ;
   
   elementSelector
  -    : '[' elementsSpec ']'
  +    : '?'? '[' elementsSpec ']'
       ;
   
   elementsSpec
  
positive.code:     | aliasDeclaration
      ;
  
  importDeclaration  
      : 'import' importElement ('.' importElement)* ('.' '*' | 'alias' typeName)? ';'
      ;
      
  importElement
  
negative.code: }
  
  compilationUnit
      : importDeclaration*
        (annotations? typeDeclaration)+
        EOF
      ;
  