exit codes

[pazz]

It'd be convenient to encourage different outcomes, especially failures, into exit codes.

For example,

• 0: all good and the given instance is positive.
• -1: all good but the given instance is negative
• 1: input could not be read
• 2: input could not be parsed into an automaton

This way, one could fit instance use the new output argument -o that writes winning strategies to a file, say as tex, and then only compile this in case the output was written, using the shell's && operator.