The real issue: interopability, and a proposal (Was: Consensus Call on Latin Sharp S and Greek Final Sigma)

[Alexander Mayrhofer]

> To be complete, one would ban mapping not only of PVALID, but 
> also of DISALLOWED (since those are in an indeterminate 
> state, and could become PVALID).

I strongly oppose that. 

Disallowing mappings of DISALLOWED characters (sic) would mean that user input in the form of "Äpfel.at" (apples) could not be mapped to "äpfel.at", because "Ä" (like any other uppercase non-ASCII character) is DISALLOWED. 

I think that the very objective of mappings is to deal with DISALLOWED characters. Not allowing that would essentially remove the purpose of the mapping itself.

Alex