Add new typeclass, Reducible

This defines a typeclass `Reducible` which allows us to "reduce" pandoc
Inlines and Blocks, like so

    Emph [Strong [Str "foo", Space]] <++> Strong [Emph [Str "bar"]], Str
"baz"] =
        [Strong [Emph [Str "foo", Space, Str "bar"], Space, Str "baz"]]

So adjacent formattings and strings are appropriately grouped.

Another set of operators for `(Reducible a) => (Many a)` are also
included.

src/Text/Pandoc/Readers/Docx/Reducible.hs

@@ -0,0 +1,150 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Text.Pandoc.Readers.Docx.Reducible ((<++>),
+                                           (<+++>),
+                                           Reducible,
+                                           Container(..),
+                                           container,
+                                           innards,
+                                           reduceList,
+                                           reduceListB,
+                                           rebuild)
+       where
+
+import Text.Pandoc.Builder
+import Data.List ((\\), intersect)
+
+data Container a = Container ([a] -> a) | NullContainer
+
+instance (Eq a) => Eq (Container a) where
+  (Container x) == (Container y) = ((x []) == (y []))
+  NullContainer == NullContainer = True
+  _ == _ = False
+
+instance (Show a) => Show (Container a) where
+  show (Container x) = "Container {" ++
+                       (reverse $ drop 3 $ reverse $ show $ x []) ++
+                       "}"
+  show (NullContainer) = "NullContainer"
+
+class Reducible a where
+  (<++>) :: a -> a -> [a]
+  container :: a -> Container a
+  innards :: a -> [a]
+  isSpace :: a -> Bool
+
+(<+++>) :: (Reducible a) => Many a -> Many a -> Many a
+mr <+++> ms = fromList $ reduceList $ toList mr ++ toList ms
+
+reduceListB :: (Reducible a) => Many a -> Many a
+reduceListB = fromList . reduceList . toList
+
+reduceList' :: (Reducible a) => [a] -> [a] -> [a]
+reduceList' acc [] = acc
+reduceList' [] (x:xs) = reduceList' [x] xs
+reduceList' as (x:xs) = reduceList' (init as ++ (last as <++> x) ) xs
+
+reduceList :: (Reducible a) => [a] -> [a]
+reduceList = reduceList' []
+
+combineReducibles :: (Reducible a, Eq a) => a -> a -> [a]
+combineReducibles r s =
+  let (conts, rs) = topLevelContainers r
+      (conts', ss) = topLevelContainers s
+      shared = conts `intersect` conts'
+      remaining = conts \\ shared
+      remaining' = conts' \\ shared
+  in
+   case null shared of
+       True -> case (not . null) rs && isSpace (last rs) of
+         True -> rebuild conts (init rs) ++ [last rs, s]
+         False -> [r,s]
+       False -> rebuild 
+                shared $
+                reduceList $
+                (rebuild remaining rs) ++ (rebuild remaining' ss)
+
+instance Reducible Inline where
+  s1@(Span (id1, classes1, kvs1) ils1) <++> s2@(Span (id2, classes2, kvs2) ils2) =
+    let classes'  = classes1 `intersect` classes2
+        kvs'      = kvs1 `intersect` kvs2
+        classes1' = classes1 \\ classes'
+        kvs1'     = kvs1 \\ kvs'
+        classes2' = classes2 \\ classes'
+        kvs2'     = kvs2 \\ kvs'
+    in
+     case null classes' && null kvs' of
+       True -> [s1,s2]
+       False -> let attr'  = ("", classes', kvs')
+                    attr1' = (id1, classes1', kvs1')
+                    attr2' = (id2, classes2', kvs2')
+                    s1' = case null classes1' && null kvs1' of
+                      True -> ils1
+                      False -> [Span attr1' ils1]
+                    s2' = case null classes2' && null kvs2' of
+                      True -> ils2
+                      False -> [Span attr2' ils2]
+                in
+                 [Span attr' $ reduceList $ s1' ++ s2']
+
+  (Str x) <++> (Str y) = [Str (x++y)]
+  il <++> il' = combineReducibles il il'
+
+  container (Emph _) = Container Emph
+  container (Strong _) = Container Strong
+  container (Strikeout _) = Container Strikeout
+  container (Subscript _) = Container Subscript
+  container (Superscript _) = Container Superscript
+  container (Quoted qt _) = Container $ Quoted qt
+  container (Cite cs _) = Container $ Cite cs
+  container (Span attr _) = Container $ Span attr
+  container _ = NullContainer
+
+  innards (Emph ils) = ils
+  innards (Strong ils) = ils
+  innards (Strikeout ils) = ils
+  innards (Subscript ils) = ils
+  innards (Superscript ils) = ils
+  innards (Quoted _ ils) = ils
+  innards (Cite _ ils) = ils
+  innards (Span _ ils) = ils
+  innards _ = []
+
+  isSpace Space = True
+  isSpace _     = False
+
+instance Reducible Block where
+  (Div (ident, classes, kvs) blks) <++> blk | "list-item" `elem` classes = 
+    [Div (ident, classes, kvs) (reduceList blks), blk]
+
+  blk <++> blk' = combineReducibles blk blk'
+
+  container (BlockQuote _) = Container BlockQuote
+  container (Div attr _) = Container $ Div attr
+  container _            = NullContainer
+
+  innards (BlockQuote bs) = bs
+  innards (Div _ bs) = bs
+  innards _          = []
+
+  isSpace _          = False
+
+
+topLevelContainers' :: (Reducible a) => [a] -> ([Container a], [a])
+topLevelContainers' (r : []) = case container r of
+  NullContainer -> ([], [r])
+  _             ->
+    let (conts, inns) = topLevelContainers' (innards r)
+    in
+    ((container r) : conts, inns)
+topLevelContainers' rs = ([], rs)
+
+topLevelContainers :: (Reducible a) => a -> ([Container a], [a])
+topLevelContainers il = topLevelContainers' [il]
+
+rebuild :: [Container a] -> [a] -> [a]
+rebuild [] xs = xs
+rebuild ((Container f) : cs) xs = rebuild cs $ [f xs]
+rebuild (NullContainer : cs) xs = rebuild cs $ xs
+  
+