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