assertFalse( children.remove( otherChild ) );
assertFalse( children.isDirty() );
HashSet otherSet = new HashSet();
otherSet.add( child );
assertFalse( children.addAll( otherSet ) );
assertFalse( children.isDirty() );
assertFalse( children.retainAll( otherSet ) );
assertFalse( children.isDirty() );