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Full Idea
If a frame contains any dead end or blind world, then D is not valid on that frame, ...but D is valid on every serial frame.
Gist of Idea
D is valid on every serial frame, but not where there are dead ends
Source
Max J. Cresswell (Modal Logic [2001], 7.1.1)
Book Ref
'Blackwell Guide to Philosophical Logic', ed/tr. Goble,Lou [Blackwell 2001], p.140