i don't understand this exception

[Skaperen]

i was already trying to write a test. it didn't support any compression and just did what _zio does for an uncompressed file. so i just let _zio be the test. i'll try, again, and make something that doesn't have the mode and option logic. or the extra function layers (their only purpose is to have function names serve as different defaults for the tempname= option since i have code around here that uses both features by name. i really should take ztopen() out since i added the tempname= option and eliminate the function layers and rename _zio to zopen.