def removableFieldsMirror :=
  mirror: {
    // vectors are not loaded with these unit tests
    // we therefore assume only a single field is removed
    def removedField := nil;
    def removeField( symbol ) {
      removedField := symbol;
    };
    def invokeField( receiver, symbol ) {
      if: (symbol == removedField) then: {
        raise: doesNotUnderstandX.new(symbol , self);
      } else: {
        super.invokeField( receiver, symbol );
      }
    }
  };
  
def test := object: {
  def visible := nil
} mirroredBy: removableFieldsMirror;

(reflect: test).removeField(`visible);
(test.visible == nil).ifTrue: { fail(); };