[relax-ng] Draft requirements for identity constraints

[John Cowan <cowan@mercury.ccil.org>]

Here is a strawman set of requirements.  I don't necessarily support
all of these myself.

"Module".  The identity-constraint system should be an optional
module expressed by a RELAX NG annotation.

"Same".  The identity-constraint system allows one to specify that
certain portions of the document are the same (a key reference is
the same as a key, e.g.)

"Different".  The identity-constraint system allows one to specify
that certain portions of the document are different (no two keys
are alike, e.g.)

"Typed".  Keys and references may have types.  Sameness and differentness
are defined as type equality and inequality.

"Anywhere".  Either element content or an attribute value may constitute
a key or a reference.

"Scoped".  The uniqueness of keys may be specified local to a particular
part of the document.

"Multiple".  Multiple different non-intersecting key spaces must be
supported.

"Split".  A key may comprise multiple different elements or attributes.

"Required".  The distinction between keys which are required and
those which are not must be supported.


-- 
John Cowan           http://www.ccil.org/~cowan              cowan@ccil.org
To say that Bilbo's breath was taken away is no description at all.  There
are no words left to express his staggerment, since Men changed the language
that they learned of elves in the days when all the world was wonderful.
        --_The Hobbit_