- Certain fairly simple data structures can only be achieved in XSD (if
  at all) with ridiculously verbose patterns; for instance, how do you
  represent a structure with five possible elements, at least one of
  which must be present and none of which may be repeated?  The best
  method I have found takes about 25 lines and far too much code
  repetition. 
...

- It [rng]  does not appear to relieve the complexity of representing some
   types of data structures (see the one described above-as far as I can
   tell, it would not be significantly easier to define in Relax NG than
   it is in XSD).


If I understand correctly, in relaxNG, something like:



start = x

x = element x {
   ( a  & b? & c? & d? & e?) |
   ( a? & b  & c? & d? & e?) |
   ( a? & b? & c  & d? & e?) |
   ( a? & b? & c? & d  & e?) |
   ( a? & b? & c? & d? & e ) 
 }

a = element a {empty}
b = element b {empty}
c = element c {empty}
d = element d {empty}
e = element e {empty}


does what you want, it validates

<x>
 <a/>
 <b/>
</x>

and invalidates

<x>
 <b/>
 <a/>
 <b/>
</x>


- Relax NG provides no way of specifying cardinalities other than 0, 1,
  and infinity!  This is also a big one for me--if I need at least 2 and
  no more than 20 occurrences of the foo element (fairly common in my
  data definitions), there's NO WAY I'm going to define two required foo
  elements and eighteen optional ones!


usually you can persuade yourself that 1,?,+,* is enough and leave
cardinality checking to higher up the chain, but if you really need
this, then powers of two come in handy:-)


y = element y { z,z,z16,z2}
z16 = z8,z8
z8  = z4,z4
z4  = z2,z2
z2  = z?,z?
z = element z {empty}

validates
<y><z/><z/></y>

but invalidates instances with less than 1 or more than 20 zs.

david

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