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