Just noticed a significant and potentially confusion causing error regarding multiply modal logics in this nonetheless excellent text. On pages 217-218 in my 1996 edition Hughes and Cresswell write:
For instance we might have a necessity operator
, say, which is stronger than
in the sense that
. The canonical model for such a system would obey the restriction that for all
if
then
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Now, that should be for all if
then
.