Undefined elements XSD definition

hi !

I would like to write a schema for this kind of XML document

<request>
<params>
<param1>foo</param1>
<pouet>foo</pouet>
<truc>bidule</truc>
</params>
<body>
.....
</body>
</request>

The number and the name of the params are undefined. Another valid XML
document could be

<request>
<params>
<azerty>foo</azerty>
<qwerty>foo</qwerty>
<name>oziris</name>
</params>
<body>
.....
</body>
</request>

Is there a XSD mean to specify that?
Using <xs:any> was my first idea but I wonder if there is better
answer.

Thanks a lot.

-o--

.