anchor.diff:      | 'interface' 
       | 'class' 
       | 'object'
  -    | 'choice'
       | 'alias'
       ;
   
  
positive.code:         satisfiedTypes?
          typeConstraints?
          classBody
      ;
  
  unionDeclaration
      :
          'choice'!
          typeName
          typeParameters?
          caseTypes
          satisfiedTypes?
          typeConstraints?
          ';'
      ;
  
  objectDeclaration
  
negative.code:     -> ^(ALIAS_DECL aliasDeclaration)
      | objectDeclaration
      -> ^(OBJECT_DECL objectDeclaration)
      ;
  
  importDeclaration
  