[relaxng-user] A question on content models

[James Clark]

Murata-san has mostly covered this already but since I had my reply 
mostly written already I'll send it as well.

> What's the difference between empty and
> notAllowed in this context?

The following analogy may help (or it may just confuse things!): if you 
think of choice as addition and interleave as multiplication, then 
notAllowed is like 0 and empty is like 1.

Whether to use notAllowed or empty in this context depends on whether 
you are going to use combine="choice" or combine="interleave". If 
combine="choice" then use notAllowed; if combine="interleave" then use 
empty.  That's because X | notAllowed is equivalent to X for any pattern 
X; similarly, X & empty is equivalent to X for any pattern X.

>>>Individual files usually say something like
>>>  <define combine="choice" name="class.reader">
>>>    <element name="switch">
>>>      <!-- CM -->
>>>    </element>
>>>  </define>
>>
>>Then use combine="interleave" instead of combine="choice".
> 
> 
> For attributes or in general? Again, this may be a FAQ, what's the
> difference between "choice" and "interleave" in this context? Is
> 
> <define name="a" combine="interleave">
>   <!-- -->
> </define>
> 
> the same as
> 
> <oneOrMore>
>   <define name="a" combine="choice">
>     <!--  -->
>   </define>
> </oneOrMore>

No. The combine attribute just says what happens when you have two 
definitions with the same name.  My fingers are getting tired, so I'm 
going to use the compact syntax here:

a &= X
a &= Y

is the same as

a = X & Y

Also

a = attribute foo { ... } & attribute bar { ... }

is equivalent to

a = attribute foo { ... }, attribute bar { ... }

(because attribute order is not significant).  So if you are defining 
sets of attributes you want to use empty and &=.  You would use 
notAllowed and |= for attributes only if you had a case where you wanted 
the schema allowed one of a class of attributes (which is rare).

James