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Custom folds and traversals of subparts.
Bugfix: genFoldMap combine order is unexpected

foldr (<>) a [b,c,d]  == b <> c <> d <> a
foldr1 (<>) [a,b,c,d] == a <> b <> c <> d
Merge pull request #1 from vklj/master

Customizable folds and traversals; some fixes