modelling the business rule "at least one of the elements"

Hi,

I'm struggling with designing an XML Schema that has a conditional
part to it.  Here's my issue:

I would like to represent data, let's call it "parent" which can
contain "at least" one of 3 elements ("child1",  "anotherchild", and
"yetanotherchild")

For example, this would be a valid "parent" element:
<parent>
    <child1>value of child 1</child1>
    <anotherchild>value of anotherchild</anotherchild>
    <yetanotherchild>value of yetanotherchild</yetanotherchild>
</parent>

This would be a valid "parent" element:
<parent>
    <anotherchild>value of anotherchild</anotherchild>
</parent>

This would be a valid "parent" element:
<parent>
    <child1>value of child 1</child1>
    <yetanotherchild>value of yetanotherchild</yetanotherchild>
</parent>

This would be a INVALID "parent" element because there should be at
least one of the child elements:
<parent>
</parent>

This would be a INVALID "parent" element because the parent element
can only contain at most 1 occurence of each kind of child element.
<parent>
    <child1>value of child 1</child1>
    <yetanotherchild>value of yetanotherchild</yetanotherchild>
    <yetanotherchild>value of yetanotherchild</yetanotherchild>
</parent>

Thanks in advance,

Joen Moreno