    {-# LANGUAGE ExistentialQuantification #-}

    -- | The class 'XmlContent' is a kind of replacement for Read and Show:
    --   it provides conversions between a generic XML tree representation
    --   and your own more specialised typeful Haskell data trees.
    --
    --   If you are starting with a set of Haskell datatypes, use DrIFT to
    --   derive instances of this class for you:
    --       http:\/\/repetae.net\/john\/computer\/haskell\/DrIFT
    --   If you are starting with an XML DTD, use HaXml's tool DtdToHaskell
    --   to generate both the Haskell types and the corresponding instances.
    --
    --   This unified class interface replaces two previous (somewhat similar)
    --   classes: Haskell2Xml and Xml2Haskell.  There was no real reason to have
    --   separate classes depending on how you originally defined your datatypes.
    --   However, some instances for basic types like lists will depend on which
    --   direction you are using.  See Text.XML.HaXml.XmlContent and
    --   Text.XML.HaXml.XmlContent.Haskell.

    --   The methods 'toContents' and 'parseContents' convert a value to and from
    --   a generic internal representation of an XML document /without/ a DTD.
    --   The functions 'toXml' and 'fromXml' convert a value to and from a generic
    --   internal representation of an XML document /including/ a DTD.
    --   The functions 'readXml' and 'showXml' convert to and from Strings.
    --   The functions 'fReadXml' and 'fWriteXml' do the conversion to and from
    --   the given filenames.
    --   The functions 'hGetXml' and 'hPutXml' do the conversion to and from
    --   the given file handles.
    --   (See the type signatures.)
    --

    module Text.XML.HaXml.XmlContent.Parser
      ( -- * Re-export the relevant set of generic XML document type definitions
        Document(..)
      , Element(..)
      , ElemTag(..)
      , Content(..)
      , Attribute()
      , AttValue(..)
      , Prolog(..)
      , Reference(..)
      -- * The enabling classes, that define parsing\/unparsing between Haskell
      --   datatypes and a generic XML representation.
      , XmlContent(..)
      , XmlAttributes(..)
      , XmlAttrType(..)
      -- ** Auxiliaries for writing parsers in the XmlContent class
      , module Text.ParserCombinators.Poly
      , XMLParser
      , content, posnElement, element, interior, inElement, text, attributes
      , posnElementWith, elementWith, inElementWith
      , choice, definite -- ???
      -- ** Auxiliaries for generating in the XmlContent class
      , mkElem, mkElemC, mkAttr
      , toText, toCData
      -- ** Auxiliaries for the attribute-related classes
      , maybeToAttr, defaultToAttr
      , definiteA, defaultA, possibleA, fromAttrToStr, toAttrFrStr
      , Defaultable(..)
      , str2attr, attr2str, attval
      , catMaybes   -- re-exported from Maybe
      -- * Explicit representation of Haskell datatype information
      --   (for conversion to a DTD)
      , module Text.XML.HaXml.TypeMapping
      -- * Types useful for some content models
      , List1(..)
      , ANYContent(..)
      ) where

    --import IO
    import Maybe (catMaybes)
    import Char  (chr, isSpace)
    import List  (isPrefixOf, isSuffixOf)

    import Text.PrettyPrint.HughesPJ (render)
    import qualified Text.XML.HaXml.Pretty as PP

    import Text.XML.HaXml.Types
    import Text.XML.HaXml.TypeMapping
    import Text.XML.HaXml.Posn     (Posn, posInNewCxt)
    import Text.XML.HaXml.Pretty   (document)
    import Text.XML.HaXml.Parse    (xmlParse)
    import Text.XML.HaXml.Verbatim (Verbatim(verbatim))
    import qualified Text.XML.HaXml.ByteStringPP as FPS (document)
    import qualified Data.ByteString.Lazy.Char8 as FPS

    import Text.ParserCombinators.Poly

    --  #define DEBUG

    #if defined(DEBUG)
    import Debug.Trace(trace)
    debug :: a -> String -> a
    v `debug` s = trace s v
    #else
    debug :: t -> t1 -> t
    v `debug` _ = v
    #endif

    ------------------------------------------------------------------------
    -- | Read a single attribute called "value".
    attval :: (Read a) => Element i -> a
    attval (Elem _ [("value",v@(AttValue _))] []) = read (show v)

    -- | Generate a single attribute.
    mkAttr :: String -> String -> Attribute
    mkAttr n v = (n, AttValue [Left v])

    -- | Generate an element with no attributes, named for its HType.
    mkElem :: XmlContent a => a -> [Content ()] -> Content ()
    mkElem x cs  = CElem (Elem (showHType (toHType x) "") [] cs) ()

    -- | Generate an element with no attributes, named directly.
    mkElemC :: String -> [Content ()] -> Content ()
    mkElemC x cs = CElem (Elem x [] cs) ()

    -- | Turn a simple string into XML text.
    toText :: String -> [Content ()]
    toText s = [CString False s ()]

    -- | Turn a string into an XML CDATA section.
    --   (i.e. special characters like '&' are preserved without interpretation.)
    toCData :: String -> [Content ()]
    toCData s = [CString True s ()]

    ------------------------------------------------------------------------

    -- | We need a parsing monad for reading generic XML Content into specific
    --   datatypes.  This is a specialisation of the Text.ParserCombinators.Poly
    --   ones, where the input token type is fixed as XML Content.
    type XMLParser a = Parser (Content Posn) a

    ------------------------------------------------------------------------
    -- Some useful parsing combinators
    ------------------------------------------------------------------------

    -- | The most primitive combinator for XMLParser - get one content item.
    content :: String -> XMLParser (Content Posn)
    content word = next `adjustErr` (++" when expecting "++word)

    -- | Get the next content element, checking that it has one of the required
    --   tags, using the given matching function.
    --   (Skips over comments and whitespace, rejects text and refs.
    --    Also returns position of element.)
    posnElementWith :: (String->String->Bool) -> [String]
                       -> XMLParser (Posn, Element Posn)
    posnElementWith match tags = do
        { c <- content (formatted tags)
        ; case c of
              CElem e@(Elem t _ _) pos
                  | any (match t) tags -> return (pos, e)
                  | otherwise          -> fail ("Found a <"++t++">, but expected "
                                               ++formatted tags++"\nat "++show pos)
              CString b s pos
                  | not b && all isSpace s -> posnElementWith match tags
                                                            -- ignore blank space
                  | otherwise -> fail ("Found text content, but expected "
                                      ++formatted tags++"\ntext is: "++s
                                      ++"\nat "++show pos)
              CRef r pos -> fail ("Found reference, but expected "
                                 ++formatted tags++"\nreference is: "++verbatim r
                                 ++"\nat "++show pos)
              CMisc _ _ -> posnElementWith match tags  -- skip comments, PIs, etc.
        }
      where
        formatted [t]  = "a <"++t++">"
        formatted tgs = "one of"++ concatMap (\t->" <"++t++">") tgs

    -- | A specialisation of @posnElementWith (==)@.
    posnElement :: [String] -> XMLParser (Posn, Element Posn)
    posnElement = posnElementWith (==)

    -- | Get the next content element, checking that it has one of the required
    --   tags.  (Skips over comments and whitespace, rejects text and refs.)
    element :: [String] -> XMLParser (Element Posn)
    element tags = fmap snd (posnElement tags)
                                    `debug` ("Element: "++unwords tags++"\n")

    -- | Like element, only permits a more flexible match against the tagname.
    elementWith :: (String->String->Bool) -> [String] -> XMLParser (Element Posn)
    elementWith match tags = fmap snd (posnElementWith match tags)
                                    `debug` ("Element: "++unwords tags++"\n")

    -- | Run an XMLParser on the contents of the given element (i.e. not on the
    --   current monadic content sequence), checking that the contents are
    --   exhausted, before returning the calculated value within the current
    --   parser context.
    interior :: Element Posn -> XMLParser a -> XMLParser a
    interior (Elem e _ cs) p =
        case runParser p cs of
            (Left msg, _) -> fail msg
            (Right x, []) -> return x
            (Right x, ds@(d:_))
                | all onlyMisc ds -> return x
                | otherwise       -> fail ("Too many elements inside <"++e++"> at\n"
                                          ++show (info d)++"\n"
                                          ++"Found excess: "++verbatim d)
      where onlyMisc (CMisc _ _) = True
            onlyMisc (CString False s _) | all isSpace s = True
            onlyMisc _ = False

    -- | A combination of element + interior.
    inElement :: String -> XMLParser a -> XMLParser a
    inElement tag p = do { e <- element [tag]; commit (interior e p) }

    -- | A combination of elementWith + interior.
    inElementWith :: (String->String->Bool) -> String -> XMLParser a -> XMLParser a
    inElementWith match tag p = do { e <- elementWith match [tag]
                                   ; commit (interior e p) }

    -- | Do some parsing of the attributes of the given element
    attributes :: XmlAttributes a => Element Posn -> XMLParser a
    attributes (Elem _ as _) = return (fromAttrs as)

    -- | 'text' is a counterpart to 'element', parsing text content if it
    --   exists.  Adjacent text and references are coalesced.
    text :: XMLParser String
    text = text' []
      where text' acc =
              do { c <- content "plain text"
                 ; case c of
                     CString _ s _        -> text' (s:acc)
                     CRef (RefChar s) _   -> text' (("&#"++show s++";") :acc)
                     CRef (RefEntity s) _ -> text' (('&':s++";"):acc)
                     CMisc _ _            -> text' acc
                     CElem _ _         -> do { reparse [c] -- put it back!
                                             ; if null acc then fail "empty string"
                                               else return (concat (reverse acc))
                                             }
                 }
              `onFail` ( if null acc then fail "empty string"
                         else return (concat (reverse acc)) )

    -- | 'choice f p' means if parseContents succeeds, apply f to the result,
    --   otherwise use the continuation parser.
    choice :: XmlContent a => (a -> b) -> XMLParser b -> XMLParser b
    choice cons (P other) =
        P (\cs-> case runParser parseContents cs of
                     (Left _, _)  -> other cs
                     (Right x, cs') -> Success cs' (cons x) )

    --choice cons other = fmap cons parseContents `onFail` other

    -- | not sure this is needed now.   'definite p' previously ensured that
    --   an element was definitely present.  Now I think the monad might take
    --   care of that for us.
    definite :: XmlContent a => XMLParser a -> String -> String -> XMLParser a
    definite p inner tag = P (\cs-> case runParser p cs of
                                       (Left _, cs')   -> Failure cs' msg'
                                       (Right x, cs')  -> Success cs' x )
      where msg' = "content error: expected "++inner++" inside <"++tag
                   ++"> element\n"

    ------------------------------------------------------------------------

    -- | The @XmlContent@ class promises that an XML Content element can be
    --   converted to and from a Haskell value.
    class HTypeable a => XmlContent a where
        -- | Convert from XML to Haskell
        parseContents :: XMLParser a
        -- | Convert from Haskell to XML
        toContents    :: a -> [Content ()]

        -- | Dummy functions (for most types): used /only/ in the Char instance
        --   for coercing lists of Char into String.
        xToChar       :: a -> Char
        xFromChar     :: Char -> a
        xToChar        = error "HaXml.XmlContent.xToChar used in error"
        xFromChar      = error "HaXml.XmlContent.xFromChar used in error"

    -- | The @XmlAttributes@ class promises that a list of XML tag attributes
    --   can be converted to and from a Haskell value.
    class XmlAttributes a where
        fromAttrs :: [Attribute] -> a
        toAttrs   :: a -> [Attribute]
    -- | The @XmlAttrType@ class promises that an attribute taking an XML
    --   enumerated type can be converted to and from a Haskell value.
    class XmlAttrType a where
        fromAttrToTyp :: String -> Attribute -> Maybe a
        toAttrFrTyp   :: String -> a -> Maybe Attribute

    ------------------------------------------------------------------------
    -- Instances for some of the standard basic datatypes.
    -- Both DtdToHaskell and Haskell2Xml share these instances.
    ------------------------------------------------------------------------

    instance (XmlContent a, XmlContent b) => XmlContent (a,b) where
        toContents (a,b) = toContents a ++ toContents b
        parseContents    = do
            { a <- parseContents
            ; b <- parseContents
            ; return (a,b)
            }

    instance (XmlContent a, XmlContent b, XmlContent c) => XmlContent (a,b,c) where
        toContents (a,b,c) = toContents a ++ toContents b ++ toContents c
        parseContents = do
            { a <- parseContents
            ; b <- parseContents
            ; c <- parseContents
            ; return (a,b,c)
            }

    instance (XmlContent a, XmlContent b, XmlContent c, XmlContent d) =>
             XmlContent (a,b,c,d) where
        toContents (a,b,c,d) = toContents a ++ toContents b ++ toContents c
                               ++ toContents d
        parseContents = do
            { a <- parseContents
            ; b <- parseContents
            ; c <- parseContents
            ; d <- parseContents
            ; return (a,b,c,d)
            }

    instance ( XmlContent a, XmlContent b, XmlContent c, XmlContent d
             , XmlContent e ) =>
             XmlContent (a,b,c,d,e) where
        toContents (a,b,c,d,e) = toContents a ++ toContents b ++ toContents c
                                 ++ toContents d ++ toContents e
        parseContents = do
            { a <- parseContents
            ; b <- parseContents
            ; c <- parseContents
            ; d <- parseContents
            ; e <- parseContents
            ; return (a,b,c,d,e)
            }

    instance ( XmlContent a, XmlContent b, XmlContent c, XmlContent d
             , XmlContent e, XmlContent f ) =>
             XmlContent (a,b,c,d,e,f) where
        toContents (a,b,c,d,e,f) = toContents a ++ toContents b ++ toContents c
                                   ++ toContents d ++ toContents e ++ toContents f
        parseContents = do
            { a <- parseContents
            ; b <- parseContents
            ; c <- parseContents
            ; d <- parseContents
            ; e <- parseContents
            ; f <- parseContents
            ; return (a,b,c,d,e,f)
            }

    instance ( XmlContent a, XmlContent b, XmlContent c, XmlContent d
             , XmlContent e, XmlContent f, XmlContent g ) =>
             XmlContent (a,b,c,d,e,f,g) where
        toContents (a,b,c,d,e,f,g)
            = toContents a ++ toContents b ++ toContents c ++ toContents d
              ++ toContents e ++ toContents f ++ toContents g
        parseContents = do
            { a <- parseContents
            ; b <- parseContents
            ; c <- parseContents
            ; d <- parseContents
            ; e <- parseContents
            ; f <- parseContents
            ; g <- parseContents
            ; return (a,b,c,d,e,f,g)
            }

    instance ( XmlContent a, XmlContent b, XmlContent c, XmlContent d
             , XmlContent e, XmlContent f, XmlContent g, XmlContent h ) =>
             XmlContent (a,b,c,d,e,f,g,h) where
        toContents (a,b,c,d,e,f,g,h)
            = toContents a ++ toContents b ++ toContents c ++ toContents d
              ++ toContents e ++ toContents f ++ toContents g ++ toContents h
        parseContents = do
            { a <- parseContents
            ; b <- parseContents
            ; c <- parseContents
            ; d <- parseContents
            ; e <- parseContents
            ; f <- parseContents
            ; g <- parseContents
            ; h <- parseContents
            ; return (a,b,c,d,e,f,g,h)
            }

    instance ( XmlContent a, XmlContent b, XmlContent c, XmlContent d
             , XmlContent e, XmlContent f, XmlContent g, XmlContent h
             , XmlContent i ) =>
             XmlContent (a,b,c,d,e,f,g,h,i) where
        toContents (a,b,c,d,e,f,g,h,i)
            = toContents a ++ toContents b ++ toContents c ++ toContents d
              ++ toContents e ++ toContents f ++ toContents g ++ toContents h
              ++ toContents i
        parseContents = do
            { a <- parseContents
            ; b <- parseContents
            ; c <- parseContents
            ; d <- parseContents
            ; e <- parseContents
            ; f <- parseContents
            ; g <- parseContents
            ; h <- parseContents
            ; i <- parseContents
            ; return (a,b,c,d,e,f,g,h,i)
            }

    instance ( XmlContent a, XmlContent b, XmlContent c, XmlContent d
             , XmlContent e, XmlContent f, XmlContent g, XmlContent h
             , XmlContent i, XmlContent j ) =>
             XmlContent (a,b,c,d,e,f,g,h,i,j) where
        toContents (a,b,c,d,e,f,g,h,i,j)
            = toContents a ++ toContents b ++ toContents c ++ toContents d
              ++ toContents e ++ toContents f ++ toContents g ++ toContents h
              ++ toContents i ++ toContents j
        parseContents = do
            { a <- parseContents
            ; b <- parseContents
            ; c <- parseContents
            ; d <- parseContents
            ; e <- parseContents
            ; f <- parseContents
            ; g <- parseContents
            ; h <- parseContents
            ; i <- parseContents
            ; j <- parseContents
            ; return (a,b,c,d,e,f,g,h,i,j)
            }

    instance ( XmlContent a, XmlContent b, XmlContent c, XmlContent d
             , XmlContent e, XmlContent f, XmlContent g, XmlContent h
             , XmlContent i, XmlContent j, XmlContent k ) =>
             XmlContent (a,b,c,d,e,f,g,h,i,j,k) where
        toContents (a,b,c,d,e,f,g,h,i,j,k)
            = toContents a ++ toContents b ++ toContents c ++ toContents d
              ++ toContents e ++ toContents f ++ toContents g ++ toContents h
              ++ toContents i ++ toContents j ++ toContents k
        parseContents = do
            { a <- parseContents
            ; b <- parseContents
            ; c <- parseContents
            ; d <- parseContents
            ; e <- parseContents
            ; f <- parseContents
            ; g <- parseContents
            ; h <- parseContents
            ; i <- parseContents
            ; j <- parseContents
            ; k <- parseContents
            ; return (a,b,c,d,e,f,g,h,i,j,k)
            }

    instance ( XmlContent a, XmlContent b, XmlContent c, XmlContent d
             , XmlContent e, XmlContent f, XmlContent g, XmlContent h
             , XmlContent i, XmlContent j, XmlContent k, XmlContent l ) =>
             XmlContent (a,b,c,d,e,f,g,h,i,j,k,l) where
        toContents (a,b,c,d,e,f,g,h,i,j,k,l)
            = toContents a ++ toContents b ++ toContents c ++ toContents d
              ++ toContents e ++ toContents f ++ toContents g ++ toContents h
              ++ toContents i ++ toContents j ++ toContents k ++ toContents l
        parseContents = do
            { a <- parseContents
            ; b <- parseContents
            ; c <- parseContents
            ; d <- parseContents
            ; e <- parseContents
            ; f <- parseContents
            ; g <- parseContents
            ; h <- parseContents
            ; i <- parseContents
            ; j <- parseContents
            ; k <- parseContents
            ; l <- parseContents
            ; return (a,b,c,d,e,f,g,h,i,j,k,l)
            }

    instance ( XmlContent a, XmlContent b, XmlContent c, XmlContent d
             , XmlContent e, XmlContent f, XmlContent g, XmlContent h
             , XmlContent i, XmlContent j, XmlContent k, XmlContent l
             , XmlContent m ) =>
             XmlContent (a,b,c,d,e,f,g,h,i,j,k,l,m) where
        toContents (a,b,c,d,e,f,g,h,i,j,k,l,m)
            = toContents a ++ toContents b ++ toContents c ++ toContents d
              ++ toContents e ++ toContents f ++ toContents g ++ toContents h
              ++ toContents i ++ toContents j ++ toContents k ++ toContents l
              ++ toContents m
        parseContents = do
            { a <- parseContents
            ; b <- parseContents
            ; c <- parseContents
            ; d <- parseContents
            ; e <- parseContents
            ; f <- parseContents
            ; g <- parseContents
            ; h <- parseContents
            ; i <- parseContents
            ; j <- parseContents
            ; k <- parseContents
            ; l <- parseContents
            ; m <- parseContents
            ; return (a,b,c,d,e,f,g,h,i,j,k,l,m)
            }

    instance ( XmlContent a, XmlContent b, XmlContent c, XmlContent d
             , XmlContent e, XmlContent f, XmlContent g, XmlContent h
             , XmlContent i, XmlContent j, XmlContent k, XmlContent l
             , XmlContent m, XmlContent n ) =>
             XmlContent (a,b,c,d,e,f,g,h,i,j,k,l,m,n) where
        toContents (a,b,c,d,e,f,g,h,i,j,k,l,m,n)
            = toContents a ++ toContents b ++ toContents c ++ toContents d
              ++ toContents e ++ toContents f ++ toContents g ++ toContents h
              ++ toContents i ++ toContents j ++ toContents k ++ toContents l
              ++ toContents m ++ toContents n
        parseContents = do
            { a <- parseContents
            ; b <- parseContents
            ; c <- parseContents
            ; d <- parseContents
            ; e <- parseContents
            ; f <- parseContents
            ; g <- parseContents
            ; h <- parseContents
            ; i <- parseContents
            ; j <- parseContents
            ; k <- parseContents
            ; l <- parseContents
            ; m <- parseContents
            ; n <- parseContents
            ; return (a,b,c,d,e,f,g,h,i,j,k,l,m,n)
            }

    instance ( XmlContent a, XmlContent b, XmlContent c, XmlContent d
             , XmlContent e, XmlContent f, XmlContent g, XmlContent h
             , XmlContent i, XmlContent j, XmlContent k, XmlContent l
             , XmlContent m, XmlContent n, XmlContent o ) =>
             XmlContent (a,b,c,d,e,f,g,h,i,j,k,l,m,n,o) where
        toContents (a,b,c,d,e,f,g,h,i,j,k,l,m,n,o)
            = toContents a ++ toContents b ++ toContents c ++ toContents d
              ++ toContents e ++ toContents f ++ toContents g ++ toContents h
              ++ toContents i ++ toContents j ++ toContents k ++ toContents l
              ++ toContents m ++ toContents n ++ toContents o
        parseContents = do
            { a <- parseContents
            ; b <- parseContents
            ; c <- parseContents
            ; d <- parseContents
            ; e <- parseContents
            ; f <- parseContents
            ; g <- parseContents
            ; h <- parseContents
            ; i <- parseContents
            ; j <- parseContents
            ; k <- parseContents
            ; l <- parseContents
            ; m <- parseContents
            ; n <- parseContents
            ; o <- parseContents
            ; return (a,b,c,d,e,f,g,h,i,j,k,l,m,n,o)
            }

    ------------------------------------------------------------------------
    -- Useful auxiliaries for "fromAttributes"
    ------------------------------------------------------------------------

    -- | If an attribute is defaultable, then it either takes the default
    --   value (which is omitted from the output), or a non-default value
    --   (which obviously must be printed).
    data Defaultable a  = Default a    | NonDefault a    deriving (Eq,Show)

    searchMaybe :: (a -> Maybe b) -> [a] -> Maybe b
    searchMaybe _ [] = Nothing
    searchMaybe f (x:xs) =
        let fx = f x in
        case fx of
          Nothing  -> searchMaybe f xs
          (Just _) -> fx

    maybeToAttr :: (String->a->Maybe Attribute) -> String -> Maybe a
                   -> Maybe Attribute
    maybeToAttr _ _ Nothing  = Nothing
    maybeToAttr to n (Just v) = to n v

    defaultToAttr :: (String->a->Maybe Attribute) -> String -> Defaultable a
                     -> Maybe Attribute
    defaultToAttr _ _ (Default _)  = Nothing
    defaultToAttr to n (NonDefault v) = to n v

    definiteA :: (String->Attribute->Maybe a) -> String -> String
                 -> [Attribute] -> a
    definiteA from tag at as =
        case searchMaybe (from at) as of
          Nothing  -> error ("missing attribute "++at++" in tag <"++tag++">")
          (Just a) -> a

    defaultA :: (String->Attribute->Maybe a) -> a -> String
                -> [Attribute] -> Defaultable a
    defaultA from def at as =
        case searchMaybe (from at) as of
          Nothing  -> Default def
          (Just a) -> NonDefault a

    possibleA :: (String->Attribute->Maybe a) -> String -> [Attribute] -> Maybe a
    possibleA from at as = searchMaybe (from at) as

    fromAttrToStr :: String -> Attribute -> Maybe String
    fromAttrToStr n (n0,v)
            | n == n0   = Just (attr2str v)
            | otherwise = Nothing

    toAttrFrStr   :: String -> String -> Maybe Attribute
    toAttrFrStr n v = Just (n, str2attr v)

    str2attr :: String -> AttValue
    str2attr s =
        let f t =
              let (l,r) = span (\c-> not (elem c "\"&<>'")) t
              in if null r then [Left l]
                 else Left l: Right (g (head r)): f (tail r)
            g '"'  = RefEntity "quot"
            g '&'  = RefEntity "amp"
            g '<'  = RefEntity "lt"
            g '>'  = RefEntity "gt"
            g '\'' = RefEntity "apos"
        in AttValue (f s)

    attr2str :: AttValue -> String          -- really needs symbol table
    attr2str (AttValue xs) =
        let f (Left s) = s
            f (Right (RefChar i))        = [chr i]
            f (Right (RefEntity "quot")) = "\""
            f (Right (RefEntity "amp"))  = "&"
            f (Right (RefEntity "lt"))   = "<"
            f (Right (RefEntity "gt"))   = ">"
            f (Right (RefEntity "apos")) = "'"
            f (Right _)                  = "*"  -- Ooops, ST needed here.
        in concatMap f xs

    ------------------------------------------------------------------------
    --  New content-model types
    ------------------------------------------------------------------------

    {-
    data OneOf2 a b
    data OneOf3 a b c
    data OneOf4 a b c d
        ... etc are now defined (with instances) in module OneOfN.
    -}

    -- | A type corresponding to XML's ANY contentspec.
    --   It is either a list of unconverted xml 'Content'
    --   or some 'XmlContent'-able value.
    --
    -- Parsing functions (e.g. 'parseContents') will always produce 'UnConverted'.
    -- Note: The Show instance for 'UnConverted' uses 'verbatim'.
    data ANYContent = forall a . (XmlContent a, Show a) => ANYContent a
                    | UnConverted [Content Posn]

    instance Show ANYContent where
        show (UnConverted c) = "UnConverted " ++ (show $ map verbatim c)
        show (ANYContent a)  = "ANYContent " ++ (show a)

    instance Eq ANYContent where
        a == b = show a == show b

    -- | The List1 type represents lists with at least one element.
    --   It is required for DTD content models that use + as a modifier.
    data List1 a = NonEmpty [a]  deriving (Eq, Show)

    ------------------------------------------------------------------------
    --  Instances for new content-model types
    ------------------------------------------------------------------------
    instance (HTypeable a) => HTypeable (List1 a) where
        toHType m  = Defined "List1" [hx]
                             [Constr "NonEmpty" [hx] [List hx] {-Nothing-}]
                   where (NonEmpty x) = m
                         hx = toHType x
    instance (XmlContent a) => XmlContent (List1 a) where
        toContents (NonEmpty xs) = concatMap toContents xs
        parseContents = fmap NonEmpty $ many1 parseContents

    instance HTypeable ANYContent where
        toHType _      = Prim "ANYContent" "ANY"
    instance XmlContent ANYContent where
        toContents (ANYContent a)  = toContents a
        toContents (UnConverted s) = map (fmap (const ())) s
        parseContents = P (\cs -> Success [] (UnConverted cs))

    ------------------------------------------------------------------------
    --
    ------------------------------------------------------------------------