Corrigendum #9

[Karl Williamson]

I ran across this in Section 3.7.4 of
http://www.unicode.org/reports/tr36/

"Use pairs of noncharacter code points in the range FDD0..FDEF. These 
are "super" private-use characters, and are discouraged for general 
interchange. The transformation would take each nibble of a byte Y, and 
add to FDD0 and FDE0, respectively. However, noncharacter code points 
may be replaced by U+FFFD ( � ) REPLACEMENT CHARACTER by some 
implementations, especially when they use them internally. (Again, 
incoming characters must never be deleted, because that can cause 
security problems.)"

I'm not sure if this affects the calculus of the Corrigendum.