#23 handle non-matching yenc filenames

[Gerard Lanois]

2. The yEnc append situation is (usually) a known bug
where
multipart yEncs have different filenames. I would
suggest for the
meantime:

or warn "ERROR: yEnc Can't append to $outfile: $1
(might be yEnc
with differently-named multiparts)\n";

or when decoding multipart yEnc, compare current
filename to last
before appending.