1) $read($null,tn,1) makes $readn 5 here on latest version 7.76, it should make it 0 instead.
2) When you access a line number in a file that does not exist, $readn isn't set to 0 but to $lines(file):
//write -c test $+(A,$crlf,B,$crlf,C) | echo -ag > $read(test,tn,75) $readn
I think this behavior has been reported in the past, it's possible to work with it once you know but it should also make $readn 0 here.
