plea for NFKC & case-folding, suggestions for definitions

I have not had time to follow the progress of this
working group, but I have now read the latest
draft-ietf-idnabis-{defs,protocol,rationale,tables}, and I have a few
high-level comments.

 1) I am very happy that an internationalized generalization of
    preferred-syntax (host name) labels is being defined, based on the
    principle of including what's needed, rather than excluding only
    what's clearly useless/harmful.  I wanted to work on this in the
    first IDN working group, instead of or in addition to the wide-open
    IDNs of IDNA2003, but the rough consensus then was that it was not
    worth the delay it would cause.

 2) I am not persuaded that IDNAbis can avoid requiring the fundamentals
    of Nameprep: NFKC and case-folding.  More on this below.

 3) I think the approach taken in the definitions section, of building
    up the smaller concepts involved in the ACE architecture, is
    better than the approach I took in RFC 3490--referring to complex
    multi-step operations ToASCII and ToUnicode as primitives.  The
    small-concepts approach allows the reader to develop some intuition.
    I have some suggestions for more concise and rigorous definitions
    following that approach (see below).

Regarding NFKC and case-folding

I think the rationale draft is trying to have it both ways.  It says
a prefix change would be required if a label that is valid in both
IDNA2003 and IDNAbis is represented by different ASCII forms in the
two protocols.  To avoid triggering that incompatibility, it defines