Re: New Canonical Decompositions to Non-Starters

From: Richard Wordingham <>
Date: Sun, 17 Feb 2013 20:56:54 +0000

On Sun, 17 Feb 2013 10:12:26 -0800
Asmus Freytag <> wrote:

> On 2/17/2013 8:20 AM, Richard Wordingham wrote:
> > Is there any guarantee that U+E4567 will not have a
> > canonical decomposition mapping to <U+0F73 TIBETAN VOWEL SIGN II,
> > U+E4568>? If so, where is it published? I thought we had guarantees
> > that new canonical decompositions to non-starters would not be
> > created (to <U+0F71, U+0F72, U+E4568> in this case), but I cannot
> > find it. This conceivable decomposition mapping appears to wriggle
> > through a loophole because U+0F73 is a starter, i.e. has canonical
> > combining class 0.

> Let me see whether I follow that.

> If you encode a new character, it can have decomposition only if that
> decomposition also contains at least one new character. (Remember,
> all decompositions are defined to be pairs, except when they are
> singletons. If a one-t0-many mapping is desired, enough intermediate,
> partially composed characters must exist to allow this longer mapping
> to be represented as a chain of simpler mappings.)

Neither U+E4567 or U+E4568 has yet been assigned, so that does not
preclude the decomposition. If the new character is the first one, it
has to be a starter - 'Property Value
Stability', Version 2.1.0+ - and so does the first value in its
canonical decomposition mapping. The only way for a new canonical
decomposition to start with a non-starter is for the first element of
an intermediate decomposition mapping to be one of U+0F73, U+0F75 and

> Now, does it make a difference whether that required new character in
> the decomposition is the first or the second? And if it does,
> can one point to a stability guarantee where that is expressed?

> Is that what you are asking?

No. I am trying to confirm that there will never be any character but
U+0344, U+0F73, U+0F75 and U+0F81 that has a non-singleton canonical
decomposition to non-starters. The only way I see can for that to
happen is a decomposition via one of U+0F73, U+0F75 and U+0F81 such as
from U+E4567 to <U+0F73, U+E4568>, and I cannot see where this is

Received on Sun Feb 17 2013 - 15:00:53 CST

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