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--></style></head><body lang=EN-GB><div class=WordSection1><p class=MsoNormal>My apologies, I've found a better way to express it and am retracting this proposal: </p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><span style='font-family:"Courier New"'>mwhen cond a = <o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'> fold [a | cond]<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>a <> mwhen p b <> c <> mwhen q d<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'> = (fold.fold) [[a],[b|p],[c],[d|q]]<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'><o:p> </o:p></span></p><p class=MsoNormal>This also works with other folds without touching monoid newtype wrappers. I had hang-ups about bool as I don't feel like you can 'read off' the ordering, and thought mempty might be a common argument , but neither of those apply here.</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Thank you for your replies.</p></div></body></html>