XSD order for extended type?

Question

I am trying to write a schema that does not require special ordering for the extended type. I tried using the "everything" group, but it seems to resolve the "sequence".

I get this error message and got a little confused.

The entire model group should appear in the particle with {min meets} = {max meets} = 1, and this particle should be part of the pair that constitutes the {content type} of the definition of a complex type.

    <complexType name="credentialElement">
    <complexContent>
        <extension base="env:namedElement">
            <all>
                <element name="username" type="env:envString" minOccurs="1" />
                <element name="password" type="env:envString" minOccurs="1" />
                <element name="domain" type="env:envString" minOccurs="0" />
            </all>
        </extension>
    </complexContent>
</complexType>

Eric

EDIT: It seems like it's impossible. After running this through xmlint, I got another error message:

"" { } , , , "", "" ; "" .

, .