using xs:choice inside xs:all

Hi ,

I am trying for an XML Schema which would achieve the following
functionality...

[1]  Atleast one or both of "element1" or "element2" can occur
[2]  Atleast one or a combination of <element3>....<element5>
[3]  <element1> ....<element<5> can occur in any order.

<Root-element>
   <element1>a</element1>
   <element2>b</element2>
   <element3>c<element3>
   <element4>d<element4>
   <element5>e<element5>
</Root-element>

I was able to achieve [1] & [2] by using a <xs:choice maxOccurs="unbounded">
for the two sets of elements and arrange it inside a <xs:sequence> but the
functionality I want is without ordering. Any help on this is appreciated.

Regards,
Mahesh