monobit cannot handle encoding: iso-ir-68

[hackerb9]

The yaff file SuperPET APL contains a line that defines its encoding as being ISO-IR-68.

# ISO-IR-68 extended with box drawings and additional APL symbols in the C0 area
encoding: iso-ir-68

This causes monobit-convert to hang when converting it to a PNG file. (By "hang" I mean it runs using up CPU and RAM until the OOM killer on my machine stops it.)

Commenting out the encoding line "fixes" the problem, but then the information about the meaning of the character positions may be lost.