mixing childs in any order and any quantity

Hi,

I'm trying to define using XML Schema a element which content mixes several
elements, some of then multiple, that can come in any order and I wonder if
even this is possible. 

Let me clarify with a example. Consider a <book> element whose content can
be up to one <title> (minOccurs=0, maxOccurs=1), up to one <author>
(minOccurs=0, maxOccurs=1) and many <chapter> (minOccurs=0,
maxOccurs=unbounded) that comes together, but combining child tags in any
order.

Examples:

<book>
   <title>...</title>
   <chapter>...</chapter>
   <chapter>...</chapter>
   <chapter>...</chapter>
   <author>...</autor>
</book>

<book>
   <title>...</title>
   <author>...</autor>
   <chapter>...</chapter>
   <chapter>...</chapter>
   <chapter>...</chapter>
</book>

How would be the XML Schema definition for such <book> element, please?
Firstly, I though using <xs:all>, but the problem is that it only allows
elements with maxOccurs=1, so it doesn't fit with <chapter> (that has
maxOccurs=unbounded).

Anybody knows a solution or workaround, please? I'm getting crazy reviewing
the XML Schema reference for some solution, but I'm not finding anything ..
Please, any help is very welcome!

Regards,

--------------------
Fermín Galán Márquez
CTTC - Centre Tecnològic de Telecomunicacions de Catalunya
Parc Mediterrani de la Tecnologia, Av. del Canal Olímpic s/n, 08860
Castelldefels, Spain
Room 1.02
Tel : +34 93 645 29 12 
Fax : +34 93 645 29 01
Email address: fermin.galan@cttc.es 

PD. Of course I could do something like:

<book>
   <title>...</title>
   <author>...</autor>
   <chapters>
      <chapter>...</chapter>
      <chapter>...</chapter>
      <chapter>...</chapter>
   </chapters>
</book>

and define <book> as a <xs:all> of <title>, <author> and <chapters>, but I
would like to solve the problem without adding the <chapters> "grouping"
element.