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important question regarding xml-schema

From: Christian Romberg <christian.romberg@uni-rostock.de>
Date: Sat, 14 Jul 2001 13:23:38 +0200
To: www-xml-schema-comments@w3c.org
Message-id: <000201c10c57$7b29a760$0300a8c0@picasso>
Hello,

I would like to model the following in xml schema:

<!ELEMENT p (n,p?)>
<!ELEMENT n #PCDATA>
...

Two possibilities are obvious:

<element name="p" type="p_type"/>
<complexType name="p_type">
  <sequence>
    <element name="n" type="string"/>
    <element minOccurs="0" name="p" type="p_type" />
  </sequence>
</complexType>

and

<element name="p">
  <complexType>
    <sequence>
      <element name="n" type="string"/>
      <element minOccurs="0" ref="p"/>
    </sequence>
  </complexType>
</element>

In my opinion, at least one of them should be permitted by xml schema
because
otherwise xml schema would be a restriction of DTD regarding this aspect.
But after reading the xml schema specification, it seems both are not
permitted due to:

"3.8.6 Constraints on Model Group Schema Components
All model groups (see Model Groups (3.8)) must satisfy the following
constraints.

Schema Component Constraint: Model Group Correct

All of the following must be true:
...
2 Circular groups are disallowed. That is, within the {particles} of a group
there may not be at any depth a particle whose {term} is the group itself."

In both cases the <sequence> corresponds to a particle(#1) that contains a
model group(#2):

first case:
#2 contains two particles, one for the n-element (#3) and the other one for
the p-element (#4),
#4's {term} is an "Element Declaration Schema Component" (#5)
#5's {type definition} is the type definition corresponding to the contained
<complexType> (#6)
#6's type definition is besides other properties the {content type}
property, that contains in our case
the value "element-only" and a particle (#7) that is the same as #1.
So #2 contains itself at a certain depth.

second case:
nearly like the first case, except that #4's {term} has to be resolved due
to the presence of the "ref".
This is a global element declaration that corresponds to an "Element
Declaration Schema Component" (#5)
Besides that the argumentation is the same.

So the quoted restriction is applicable with the result, that both attempts
to model the given part of a DTD
are not permitted.

My question is:
Is this correct, or did I oversee something?
In case my argumentation was correct, is this the intention to not allow
such constructions?
In case I made a mistake, what production is correct and more importantly,
why?

Thanks in advance!

Yours sincerely

Christian Romberg
Received on Saturday, 14 July 2001 07:23:22 GMT

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