Multiple content descriptions

Unordered contents is a problem for regular expressions and may cause a combinatorial explosion.

Example:
"an author node must contain in sequence first, optionally initial, and then last, and somewhere in between, a homepage is required."

The DSD solution is multiple descriptions and projected contents.

Multiple content descriptions in a constraint

are checked one at a time,
only look at the contents projected onto the elements that they each mention,
each consume the content nodes that they recognize, and
at the end, each content node must be consumed once.

The following two content descriptions solve the problem in the example above:

<Content>
  <Sequence>
    <Element IDRef="first"/>
    <Optional><Element IDRef="initial"/></Optional>
    <Element IDRef="last"/>
  </Sequence>
</Content>

<Content>
  <Element IDRef="homepage"/>
</Content>

If the content sequence is first.homepage.initial.last.homepage
then the first content description consumes first..........initial.last.........
and the second consumes ......homepage......................
so the sequence is rejected (because the second homepage is not consumed).

This allows concise specification of mixed ordered and unordered content.

If the content sequence is	`first.homepage.initial.last.homepage`
then the first content description consumes	`first..........initial.last.........`
and the second consumes	`......homepage......................`
so the sequence is rejected (because the second `homepage` is not consumed).