also use □ in bi_{least,greates}_fixpoint
This is a follow-up to !714 (merged) which changed the statement of bi_mono_pred
. I propose we also change bi_{least,greates}_fixpoint to match.
This is a follow-up to !714 (merged) which changed the statement of bi_mono_pred
. I propose we also change bi_{least,greates}_fixpoint to match.