A New Introduction to Modal Logic erratum

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

Now, that should be for all if then .