SPOT_PR_CHECK=666 crashes
This is a good error message:
next ~/git/spot/bin % SPOT_PR_CHECK=foo ./ltlfilt --recurrence -f 'FGa'
ltlfilt: invalid value for SPOT_PR_CHECK (should be 1 or 2)
This is not:
next ~/git/spot/bin % SPOT_PR_CHECK=666 ./ltlfilt --recurrence -f 'FGa'
zsh: abort (core dumped) SPOT_PR_CHECK=666 ./ltlfilt --recurrence -f 'FGa'
Seems quite trivial.