default FieldDeclaration asField(){
  throw new UnsupportedOperationException(String.format("%s is not a FieldDeclaration",this));
}

default FieldDeclaration asField ( ) { throw new UnsupportedOperationException ( String . format ( "%s is not a FieldDeclaration" , this ) ) ; }
return this as a fielddeclaration or throw an unsupportedoperationexception
