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Thanks all for quick reply. My initial thoughts regarding a
fix were as below. The relevant piece of code seems to be in
function "nfc_nfkc()" in the file unicodedata.c

>           if (comb1 && comb == comb1) { 
>               /* Character is blocked. */ 
>               i1++; 
>               continue; 
>           } 

That should possibly be changed to: 

>           if (comb1 && (comb <= comb1)) { 
>               /* Character is blocked. */ 
>               i1++; 
>               continue; 
>           } 

because the new spec says "either B is a starter or it has
the same or higher combining class as C".