Punycode license

[israel shahak]

The files punycode.c and punycode.h state that "...derivative works
that comment on or otherwise explain it or assist in its
implementation may be prepared, copied, published, and
distributed...". This seems to restrict modification. There are,
however, other copyright statements, so it's not clear what this
statement applies to. If this could be clarified, I would appreciate
it.