Currently in {Ref3DPxg | Area3DPxg}.toFormulaString() each sheet name is screened with ' ' if necessary, producing results like:
[0]'1screened'!A1, [0]notScreened:'1screened'!A1, etc.

However, Excel behaves differently and screens the entire book+sheet [range] section if any sheet name needs screening. Thus formatting this part of a reference string cannot be done recursively but shall be done in one piece.

I have already written a solution and will post a pull request soon.

For now, a simple test case:

    @Test
    public void test() {
        Ref3DPxg instance = new Ref3DPxg(
                0,
                new SheetIdentifier("book", new NameIdentifier("1screened", false)),
                "A1");

        assertEquals("'[0]1screened'", instance.toFormulaString());
    }