Validating schemas with recursive types

[Jim Puccio <ch...@earthlink.net>]

I am trying to make a schema for a grammar that, in its most stripped-down
version, looks like

tree := <list> [ tree ]* </list> | <leaf/>

I am totally baffled as to how to accomplish this, though I've been able to
make one for the verbose variant

tree := <list> [ <item> tree </item> ]* </list> | <leaf/>

as follows:

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema
    xmlns:xsd = "http://www.w3.org/1999/XMLSchema"
    xmlns:xsi = "http://www.w3.org/1999/XMLSchema-instance">
    <xsd:complexType name = "tree">
        <xsd:choice>
            <xsd:element name = "node">
                <xsd:complexType>
                    <xsd:element name = "item" type = "tree" minOccurs = "0"
maxOccurs = "unbounded"/>
                </xsd:complexType>
             </xsd:element>
            <xsd:element name = "leaf"/>
        </xsd:choice>
    </xsd:complexType>
    <xsd:element name = "root" type = "tree"/>
</xsd:schema>

This does just fine parsing an instance document that looks like

<?xml version="1.0" encoding="UTF-8"?>
<root
    xmlns:xsi = "http://www.w3.org/1999/XMLSchema-instance"
    xsi:noNamespaceSchemaLocation = "adapter.xsd">
    <node>
        <item><leaf/></item>
        <item><leaf/></item>
        <item>
            <node>
                <item><leaf/></item>
            </node>
        </item>
    </node>
</root>

but all those <item>s are really obnoxious, and what we really want is to be
able to parse the terser structure

<?xml version="1.0" encoding="UTF-8"?>
<root
    xmlns:xsi = "http://www.w3.org/1999/XMLSchema-instance"
    xsi:noNamespaceSchemaLocation = "adapter.xsd">
    <node>
        <leaf/>
        <leaf/>
        <node><leaf/></node>
    </node>
</root>

instead.  I've been struggling with abstract types and equivalence classes,
trying to accomplish this, but to no avail.  Am I trying to do something
that XML has been designed to prevent?  If there is a way to do what I want,
could someone please help me to understand how to accomplish it?  The
relevant examples that I have to work from, in Box, Skonnard & Lam's book,
David Hunter's book, and the XML Schema Working Draft (Sep 2000) all fail
for one reason or another on Xerces 1.2.0, which is what I am using.

Thanks in advance,

-- Jim Puccio