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diff --git a/src/Network/DNS/RBL/Domain.hs b/src/Network/DNS/RBL/Domain.hs
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+{-# LANGUAGE DoAndIfThenElse #-}
+
+-- | The 'Domain' data type and its parser. A 'Domain' represents a
+--   name in the domain name system (DNS) as described by
+--   RFC1035. In particular, we enforce the restrictions from Section
+--   2.3.1 \"Preferred name syntax\". See for example,
+--
+--     <https://tools.ietf.org/html/rfc1035#section-2.3.1>
+--
+--   We basically work with strings and characters everywhere, even
+--   though this isn't really correct. The length specifications in
+--   the RFCs are all in terms of octets, so really a ByteString.Char8
+--   would be more appropriate. With strings, for example, we could
+--   have a unicode mumbo jumbo character that takes up two bytes
+--   (octets).
+--
+module Network.DNS.RBL.Domain (
+  UserDomain,
+  user_domain )
+where
+
+import Data.Char ( toLower )
+import Text.Parsec (
+  (<|>),
+  char,
+  optionMaybe,
+  string,
+  try )
+import qualified Text.Parsec as Parsec ( digit, letter)
+import Text.Parsec.String ( Parser )
+
+import Network.DNS.RBL.Pretty ( Pretty(..) )
+
+-- * Digits
+
+-- | A wrapper around a digit character.
+--
+newtype Digit = Digit Char deriving (Eq, Show)
+instance Pretty Digit where pretty_show (Digit d) = [d]
+
+-- | Parse a single digit, but wrap it in our 'Digit' type.
+--
+digit :: Parser Digit
+digit = fmap Digit Parsec.digit
+
+
+-- * Letters
+
+-- | A wrapper around a letter character.
+--
+newtype Letter = Letter Char deriving (Show)
+instance Pretty Letter where pretty_show (Letter l) = [l]
+
+
+-- | Parse a single letter, but wrap it in our 'Letter' type.
+--
+letter :: Parser Letter
+letter = fmap Letter Parsec.letter
+
+-- | The derived instance of 'Eq' for letters is incorrect. All
+--   comparisons should be made case-insensitively. The following
+--   is an excerpt from RFC1035:
+--
+--     2.3.3. Character Case
+--
+--     For all parts of the DNS that are part of the official
+--     protocol, all comparisons between character strings (e.g.,
+--     labels, domain names, etc.)  are done in a case-insensitive
+--     manner...
+--
+--   Since each part of DNS name is composed of our custom types, it
+--   suffices to munge the equality for 'Letter'. RFC4343
+--   <https://tools.ietf.org/html/rfc4343> clarifies the
+--   case-insensitivity rules, but the fact that we're treating DNS
+--   names as strings makes most of those problems go away (in
+--   exchange for new ones).
+--
+instance Eq Letter where
+  (Letter l1) == (Letter l2) = (toLower l1) == (toLower l2)
+
+-- * Letters/Digits
+
+-- | A sum type representing either a letter or a digit.
+--
+data LetDig =
+  LetDigLetter Letter |
+  LetDigDigit  Digit
+  deriving (Eq, Show)
+
+instance Pretty LetDig where
+  pretty_show (LetDigLetter l) = pretty_show l
+  pretty_show (LetDigDigit d) = pretty_show d
+
+-- | Parse a letter or a digit and wrap it in our 'LetDig' type.
+--
+let_dig :: Parser LetDig
+let_dig = (fmap LetDigLetter letter) <|> (fmap LetDigDigit digit)
+
+
+-- * Hyphens
+
+-- | A wrapper around a single hyphen character.
+--
+newtype Hyphen = Hyphen Char deriving (Eq, Show)
+instance Pretty Hyphen where pretty_show (Hyphen h) = [h]
+
+-- | Parse a single hyphen and wrap it in our 'Hyphen' type.
+--
+hyphen :: Parser Hyphen
+hyphen = fmap Hyphen (char '-')
+
+
+-- * Letter, Digit, or Hyphen.
+
+-- | A sum type representing a letter, digit, or hyphen.
+--
+data LetDigHyp =
+  LetDigHypLetDig LetDig |
+  LetDigHypHyphen Hyphen
+  deriving (Eq, Show)
+
+instance Pretty LetDigHyp where
+  pretty_show (LetDigHypLetDig ld) = pretty_show ld
+  pretty_show (LetDigHypHyphen h) = pretty_show h
+
+
+-- | The following is the simplest type in the domain grammar that
+--   isn't already implemented for us.
+--
+--     <let-dig> ::= <letter> | <digit>
+--
+--   ==== _Examples_
+--
+--   >>> import Text.Parsec ( parseTest )
+--
+--   Letters, digits, and hyphens are all parsed:
+--
+--   >>> parseTest let_dig_hyp "a"
+--   LetDigHypLetDig (LetDigLetter (Letter 'a'))
+--
+--   >>> parseTest let_dig_hyp "7"
+--   LetDigHypLetDig (LetDigDigit (Digit '7'))
+--
+--   >>> parseTest let_dig_hyp "-"
+--   LetDigHypHyphen (Hyphen '-')
+--
+--   However, an underscore (for example) is not:
+--
+--   >>> parseTest let_dig_hyp "_"
+--   parse error at (line 1, column 1):
+--   unexpected "_"
+--   expecting letter, digit or "-"
+--
+let_dig_hyp :: Parser LetDigHyp
+let_dig_hyp =
+  parse_letdig <|> parse_hyphen
+  where
+    parse_letdig :: Parser LetDigHyp
+    parse_letdig = fmap LetDigHypLetDig let_dig
+
+    parse_hyphen :: Parser LetDigHyp
+    parse_hyphen = fmap LetDigHypHyphen hyphen
+
+
+-- * Letter/Digit/Hyphen strings
+
+-- | A string of letters, digits, and hyphens from the RFC1035 grammar:
+--
+--     <ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
+--
+--   These are represented as either a single instance of a
+--   'LetDigHyp', or a string of them (recursive).
+--
+data LdhStr =
+  LdhStrSingleLdh LetDigHyp |
+  LdhStrMultipleLdh LetDigHyp LdhStr
+  deriving (Eq, Show)
+
+instance Pretty LdhStr where
+  pretty_show (LdhStrSingleLdh ldh) = pretty_show ldh
+  pretty_show (LdhStrMultipleLdh ldh s) = (pretty_show ldh) ++ (pretty_show s)
+
+-- | Parse a string of letters, digits, and hyphens (an 'LdhStr').
+--
+--   ==== _Examples_
+--
+--   >>> import Text.Parsec ( parseTest )
+--
+--   Single letters, digits, and hyphens are parsed:
+--
+--   >>> parseTest ldh_str "a"
+--   LdhStrSingleLdh (LetDigHypLetDig (LetDigLetter (Letter 'a')))
+--
+--   >>> parseTest ldh_str "0"
+--   LdhStrSingleLdh (LetDigHypLetDig (LetDigDigit (Digit '0')))
+--
+--   >>> parseTest ldh_str "-"
+--   LdhStrSingleLdh (LetDigHypHyphen (Hyphen '-'))
+--
+--   As well as strings of them:
+--
+--   >>> import Text.Parsec ( parse )
+--   >>> pretty_print $ parse ldh_str "" "a0-b"
+--   a0-b
+--
+ldh_str :: Parser LdhStr
+ldh_str = try both <|> just_one
+  where
+    both :: Parser LdhStr
+    both = do
+      ldh1 <- let_dig_hyp
+      ldh_tail <- ldh_str
+      return $ LdhStrMultipleLdh ldh1 ldh_tail
+
+    just_one :: Parser LdhStr
+    just_one = fmap LdhStrSingleLdh let_dig_hyp
+
+
+
+-- | A version of 'last' that works on a 'LdhStr' rather than a
+--   list. That is, it returns the last 'LetDigHyp' in the
+--   string. Since 'LdhStr' contains at least one character, there's
+--   no \"nil\" case here.
+--
+--   ==== _Examples_
+--
+--   >>> import Text.Parsec ( parse )
+--
+--   >>> let (Right r) = parse ldh_str "" "a"
+--   >>> last_ldh_str r
+--   LetDigHypLetDig (LetDigLetter (Letter 'a'))
+--
+--   >>> let (Right r) = parse ldh_str "" "abc-def"
+--   >>> last_ldh_str r
+--   LetDigHypLetDig (LetDigLetter (Letter 'f'))
+--
+last_ldh_str :: LdhStr -> LetDigHyp
+last_ldh_str (LdhStrSingleLdh x) = x
+last_ldh_str (LdhStrMultipleLdh _ x) = last_ldh_str x
+
+
+-- | A version of 'init' that works on a 'LdhStr' rather than a
+--   list. That is, it returns everything /except/ the last character in
+--   the string.
+--
+--   Since an 'LdhStr' must contain at least one character, this might
+--   not be opssible (when the input is of length one). So, we return
+--   a 'Maybe' value.
+--
+--   ==== _Examples_
+--
+--   >>> import Text.Parsec ( parse )
+--
+--   >>> let (Right r) = parse ldh_str "" "a"
+--   >>> init_ldh_str r
+--   Nothing
+--
+--   >>> let (Right r) = parse ldh_str "" "ab"
+--   >>> init_ldh_str r
+--   Just (LdhStrSingleLdh (LetDigHypLetDig (LetDigLetter (Letter 'a'))))
+--
+--   >>> let (Right r) = parse ldh_str "" "abc-def"
+--   >>> init_ldh_str r
+--   Just (LdhStrMultipleLdh (LetDigHypLetDig (LetDigLetter (Letter 'a'))) (LdhStrMultipleLdh (LetDigHypLetDig (LetDigLetter (Letter 'b'))) (LdhStrMultipleLdh (LetDigHypLetDig (LetDigLetter (Letter 'c'))) (LdhStrMultipleLdh (LetDigHypHyphen (Hyphen '-')) (LdhStrMultipleLdh (LetDigHypLetDig (LetDigLetter (Letter 'd'))) (LdhStrSingleLdh (LetDigHypLetDig (LetDigLetter (Letter 'e')))))))))
+--
+init_ldh_str :: LdhStr -> Maybe LdhStr
+init_ldh_str (LdhStrSingleLdh _) = Nothing
+init_ldh_str (LdhStrMultipleLdh h t) =
+  Just $ case (init_ldh_str t) of
+           -- We just got the second-to-last character, we're done.
+           Nothing   -> LdhStrSingleLdh h
+
+           -- There's still more stuff. Recurse.
+           Just rest -> LdhStrMultipleLdh h rest
+
+
+-- | Compute the length of an 'LdhStr'. It will be at least one, since
+--   'LdhStr's are non-empty. And if there's something other than the
+--   first character present, we simply recurse.
+--
+--   ==== _Examples_
+--
+--   >>> import Text.Parsec ( parse )
+--
+--   >>> let (Right r) = parse ldh_str "" "a"
+--   >>> length_ldh_str r
+--   1
+--
+--   >>> let (Right r) = parse ldh_str "" "abc-def"
+--   >>> length_ldh_str r
+--   7
+--
+length_ldh_str :: LdhStr -> Int
+length_ldh_str (LdhStrSingleLdh _) = 1
+length_ldh_str (LdhStrMultipleLdh _ t) = 1 + (length_ldh_str t)
+
+-- * Letter/Digit/Hyphen string followed by a trailing Letter/Digit
+
+-- | This type isn't explicitly part of the grammar, but it's what
+--   shows up in the square brackets of,
+--
+--     <label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]
+--
+--   The ldh-str is optional, but if one is present, we must also have
+--   a trailing let-dig to prevent the name from ending with a
+--   hyphen. This can be represented with a @Maybe LdhStrLetDig@,
+--   which is why we're about to define it.
+--
+data LdhStrLetDig = LdhStrLetDig (Maybe LdhStr) LetDig
+  deriving (Eq, Show)
+
+instance Pretty LdhStrLetDig where
+  pretty_show (LdhStrLetDig Nothing ld) = pretty_show ld
+  pretty_show (LdhStrLetDig (Just s) ld) = (pretty_show s) ++ (pretty_show ld)
+
+-- | Parse an 'LdhStrLetDig'. This isn't in the grammar, but we might
+--   as well define the parser for it independently since we gave it
+--   its own data type.
+--
+--   ==== _Examples_
+--
+--   >>> import Text.Parsec ( parse, parseTest )
+--
+--   Make sure we can parse a single character:
+--
+--   >>> parseTest ldh_str_let_dig "a"
+--   LdhStrLetDig Nothing (LetDigLetter (Letter 'a'))
+--
+--   And longer strings:
+--
+--   >>> pretty_print $ parse ldh_str_let_dig "" "ab"
+--   ab
+--
+--   >>> pretty_print $ parse ldh_str_let_dig "" "-b"
+--   -b
+--
+--   >>> parseTest ldh_str_let_dig "b-"
+--   parse error at (line 1, column 3):
+--   label cannot end with a hyphen
+--
+ldh_str_let_dig :: Parser LdhStrLetDig
+ldh_str_let_dig = do
+  -- This will happily eat up the trailing let-dig...
+  full_ldh <- ldh_str
+
+  -- So we have to go back and see what happened.
+  case (last_ldh_str full_ldh) of
+    (LetDigHypHyphen _) -> fail "label cannot end with a hyphen"
+    (LetDigHypLetDig ld) ->
+      -- Ok, the label didn't end with a hyphen; now we need to split
+      -- off the last letter/digit so we can pack it into our return
+      -- type separately.
+      return $ case (init_ldh_str full_ldh) of
+                 -- We only parsed one letter/digit. This can happen
+                 -- if the label contains two characters. For example,
+                 -- if we try to parse the label "ab", then the "a"
+                 -- will be eaten by the label parser, and this
+                 -- function will be left with only "b".
+                 Nothing -> LdhStrLetDig Nothing ld
+
+                 -- Usual case: there's was some leading let-dig-hyp junk,
+                 -- return it too.
+                 leading_ldhs -> LdhStrLetDig leading_ldhs ld
+
+
+
+-- | Compute the length of a 'LdhStrLetDig'. It's at least one, since
+--   the let-dig at the end is always there. And when there's an
+--   ldh-str too, we add its length to one.
+--
+--   ==== _Examples_
+--
+--   >>> import Text.Parsec ( parse )
+--
+--   >>> let (Right r) = parse ldh_str_let_dig "" "a"
+--   >>> length_ldh_str_let_dig r
+--   1
+--
+--   >>> let (Right r) = parse ldh_str_let_dig "" "abc-def"
+--   >>> length_ldh_str_let_dig r
+--   7
+--
+length_ldh_str_let_dig :: LdhStrLetDig -> Int
+length_ldh_str_let_dig (LdhStrLetDig Nothing _) = 1
+length_ldh_str_let_dig (LdhStrLetDig (Just ldhstring) _) =
+  1 + (length_ldh_str ldhstring)
+
+
+-- * Labels
+
+-- | The label type from the RFC1035 grammar:
+--
+--     <label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]
+--
+--   We allow the slightly more general syntax from RFC1123, Section 2.1:
+--
+--     The syntax of a legal Internet host name was specified in RFC-952
+--     [DNS:4].  One aspect of host name syntax is hereby changed: the
+--     restriction on the first character is relaxed to allow either a
+--     letter or a digit.  Host software MUST support this more liberal
+--     syntax.
+--
+data Label = Label Letter (Maybe LdhStrLetDig)
+  deriving (Eq, Show)
+
+instance Pretty Label where
+  pretty_show (Label l Nothing) = pretty_show l
+  pretty_show (Label l (Just s)) = (pretty_show l) ++ (pretty_show s)
+
+-- | Parse a 'Label'.
+--
+--   In addition to the grammar, there's another restriction on
+--   labels: their length must be 63 characters or less. Quoting
+--   Section 2.3.1, \"Preferred name syntax\", of RFC1035:
+--
+--     The labels must follow the rules for ARPANET host names.  They
+--     must start with a letter, end with a letter or digit, and have
+--     as interior characters only letters, digits, and hyphen.  There
+--     are also some restrictions on the length.  Labels must be 63
+--     characters or less.
+--
+--   We check this only after we have successfully parsed a label.
+--
+--   ==== _Examples_
+--
+--   >>> import Text.Parsec ( parse, parseTest )
+--
+--   Make sure we can parse a single character:
+--
+--   >>> parseTest label "a"
+--   Label (Letter 'a') Nothing
+--
+--   And longer strings:
+--
+--   >>> pretty_print $ parse label "" "abc-def"
+--   abc-def
+--
+--   But not anything ending in a hyphen:
+--
+--   >>> parseTest label "abc-"
+--   parse error at (line 1, column 5):
+--   label cannot end with a hyphen
+--
+--   Or anything over 63 characters:
+--
+--   >>> parseTest label (['a'..'z'] ++ ['a'..'z'] ++ ['a'..'z'])
+--   parse error at (line 1, column 79):
+--   labels must be 63 or fewer characters
+--
+--   However, /exactly/ 63 characters is acceptable:
+--
+--   >>> pretty_print $ parse label "" (replicate 63 'x')
+--   xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+--
+label :: Parser Label
+label = do
+  l <- letter -- Guaranteed to be there
+  maybe_s <- optionMaybe ldh_str_let_dig -- Might not be there
+  case maybe_s of
+    -- It can only be one character long, from the letter...
+    Nothing -> return $ Label l maybe_s
+
+    -- The letter gives us one character, so we check that the rest is
+    -- less than 62 characters long. But in the error message we need
+    -- to report 63.
+    Just s  -> if (length_ldh_str_let_dig s) <= 62
+              then return $ Label l maybe_s
+              else fail "labels must be 63 or fewer characters"
+
+
+
+-- * Subdomains
+
+
+-- | The data type representing a \"subdomain\" from RFC1035:
+--
+--     <subdomain> ::= <label> | <subdomain> "." <label>
+--
+--   We have reversed the order of the subdomain and label in the
+--   second option, however. This is explained in 'subdomain'.
+--
+data Subdomain =
+  SubdomainSingleLabel Label |
+  SubdomainMultipleLabel Label Subdomain
+  deriving (Eq, Show)
+
+
+
+instance Pretty Subdomain where
+  pretty_show (SubdomainSingleLabel l) = pretty_show l
+  pretty_show (SubdomainMultipleLabel l s) =
+    (pretty_show l) ++ "." ++ (pretty_show s)
+
+-- | Parse an RFC1035 \"subdomain\". The given grammar is,
+--
+--     <subdomain> ::= <label> | <subdomain> "." <label>
+--
+--   However, we have reversed the order of the subdomain and label to
+--   prevent infinite recursion. The second option (subdomain + label)
+--   is obviously more specific, we we need to try it first. This
+--   presents a problem: we're trying to parse a subdomain in terms of
+--   a subdomain! The given grammar represents subdomains how we like
+--   to think of them; from right to left. But it's better to parse
+--   from left to right, so we pick off the leading label and then
+--   recurse into the definition of subdomain.
+--
+--   According to RFC1034, Section 3.1, two neighboring labels in a
+--   DNS name cannot be equal:
+--
+--     Each node has a label, which is zero to 63 octets in length.  Brother
+--     nodes may not have the same label, although the same label can be used
+--     for nodes which are not brothers.  One label is reserved, and that is
+--     the null (i.e., zero length) label used for the root.
+--
+--   We enforce this restriction, but the result is usually that we
+--   only parse the part of the subdomain leading up to the repeated
+--   label.
+--
+--   ==== _Examples_
+--
+--   >>> import Text.Parsec ( parse, parseTest )
+--
+--   Make sure we can parse a single character:
+--
+--   >>> parseTest subdomain "a"
+--   SubdomainSingleLabel (Label (Letter 'a') Nothing)
+--
+--   >>> pretty_print $ parse subdomain "" "example.com"
+--   example.com
+--
+--   >>> pretty_print $ parse subdomain "" "www.example.com"
+--   www.example.com
+--
+--   We reject a subdomain with equal neighbors, but this leads to
+--   only the single first label being parsed instead:
+--
+--   >>> pretty_print $ parse subdomain "" "www.www.example.com"
+--   www
+--
+--   But not one with a repeated but non-neighboring label:
+--
+--   >>> pretty_print $ parse subdomain "" "www.example.www.com"
+--   www.example.www.com
+--
+subdomain :: Parser Subdomain
+subdomain = try both <|> just_one
+  where
+    both :: Parser Subdomain
+    both = do
+      l <- label
+      _ <- char '.'
+      s <- subdomain
+      let result = SubdomainMultipleLabel l s
+      if (subdomain_has_equal_neighbors result)
+      then fail "subdomain cannot have equal neighboring labels"
+      else return result
+
+    just_one :: Parser Subdomain
+    just_one = fmap SubdomainSingleLabel label
+
+
+
+-- | Retrieve a list of labels contained in a 'Subdomain'.
+--
+--   ==== _Examples_
+--
+--   >>> import Text.Parsec ( parse )
+--
+--   >>> let (Right r) = parse subdomain "" "a"
+--   >>> pretty_print $ subdomain_labels r
+--   ["a"]
+--
+--   >>> let (Right r) = parse subdomain "" "example.com"
+--   >>> pretty_print $ subdomain_labels r
+--   ["example","com"]
+--
+--   >>> let (Right r) = parse subdomain "" "www.example.com"
+--   >>> pretty_print $ subdomain_labels r
+--   ["www","example","com"]
+--
+subdomain_labels :: Subdomain -> [Label]
+subdomain_labels (SubdomainSingleLabel l) = [l]
+subdomain_labels (SubdomainMultipleLabel l s) = l : (subdomain_labels s)
+
+
+-- | Return a list of pairs of neighboring labels in a subdomain.
+--
+--   ==== _Examples_
+--
+--   >>> import Text.Parsec ( parse )
+--   >>> let (Right r) = parse subdomain "" "www.example.com"
+--   >>> pretty_print $ subdomain_label_neighbors r
+--   ["(\"www\",\"example\")","(\"example\",\"com\")"]
+--
+subdomain_label_neighbors :: Subdomain -> [(Label,Label)]
+subdomain_label_neighbors s =
+  zip ls (tail ls)
+  where
+    ls = subdomain_labels s
+
+
+-- | Return @True@ if the subdomain has any two equal neighboring
+--   labels, and @False@ otherwise.
+--
+--   ==== _Examples_
+--
+--   >>> import Text.Parsec ( parse )
+--
+--   >>> let (Right r) = parse subdomain "" "www.example.com"
+--   >>> subdomain_has_equal_neighbors r
+--   False
+--
+--   >>> let (Right l) = parse label "" "www"
+--   >>> let (Right s) = parse subdomain "" "www.example.com"
+--   >>> let bad_subdomain = SubdomainMultipleLabel l s
+--   >>> subdomain_has_equal_neighbors bad_subdomain
+--   True
+--
+subdomain_has_equal_neighbors :: Subdomain -> Bool
+subdomain_has_equal_neighbors s =
+  or [ x == y | (x,y) <- subdomain_label_neighbors s ]
+
+
+
+-- * Domains
+
+-- | An RFC1035 domain. According to RFC1035 a domain can be either a
+-- subdomain or \" \", which according to RFC2181
+-- <https://tools.ietf.org/html/rfc2181#section-11> means the root:
+--
+--      The zero length full name is defined as representing the root
+--      of the DNS tree, and is typically written and displayed as
+--      \".\".
+--
+--   We let the 'Domain' type remain true to those RFCs, even though
+--   they don't support an absolute domain name of e.g. a single dot.
+--   We have one more data type 'UserDomain' which handles the possibility
+--   of an absolute path.
+--
+data Domain =
+  DomainName Subdomain |
+  DomainRoot
+  deriving (Eq, Show)
+
+instance Pretty Domain where
+  pretty_show DomainRoot = ""
+  pretty_show (DomainName s) = pretty_show s
+
+-- | Parse an RFC1035 \"domain\"
+--
+--   ==== _Examples_
+--
+--   >>> import Text.Parsec ( parse, parseTest )
+--
+--   Make sure we can parse a single character:
+--
+--   >>> parseTest domain "a"
+--   DomainName (SubdomainSingleLabel (Label (Letter 'a') Nothing))
+--
+--   And the empty domain:
+--
+--   >>> parseTest domain ""
+--   DomainRoot
+--
+--   We will in fact parse the \"empty\" domain off the front of
+--   pretty much anything:
+--
+--   >>> parseTest domain "8===D"
+--   DomainRoot
+--
+--   Equality of domains is case-insensitive:
+--
+--   >>> let (Right r1) = parse domain "" "example.com"
+--   >>> let (Right r2) = parse domain "" "ExaMPle.coM"
+--   >>> r1 == r2
+--   True
+--
+--   A single dot IS parsed as the root, but the dot isn't consumed:
+--
+--   >>> parseTest domain "."
+--   DomainRoot
+--
+--   Anything over 255 characters is an error, so the root will be
+--   parsed:
+--
+--   >>> let big_l1 = replicate 63 'x'
+--   >>> let big_l2 = replicate 63 'y' -- Avoid equal neighboring labels!
+--   >>> let big_labels = big_l1 ++ "." ++ big_l2 ++ "."
+--   >>> let big_subdomain = concat $ replicate 3 big_labels
+--   >>> parseTest domain big_subdomain
+--   DomainRoot
+--
+--   But exactly 255 is allowed:
+--
+--   >>> import Data.List ( intercalate )
+--   >>> let l1 = replicate 63 'w'
+--   >>> let l2 = replicate 63 'x'
+--   >>> let l3 = replicate 63 'y'
+--   >>> let l4 = replicate 63 'z'
+--   >>> let big_subdomain = intercalate "." [l1,l2,l3,l4]
+--   >>> let (Right r) = parse domain "" big_subdomain
+--   >>> length (pretty_show r)
+--   255
+--
+domain :: Parser Domain
+domain = try parse_subdomain <|> parse_empty
+  where
+    parse_subdomain :: Parser Domain
+    parse_subdomain = do
+      s <- subdomain
+      if length (pretty_show s) <= 255
+      then return $ DomainName s
+      else fail "subdomains can be at most 255 characters"
+
+    parse_empty :: Parser Domain
+    parse_empty = string "" >> return DomainRoot
+
+
+
+-- * User domains
+
+-- | This type helps clarify some murkiness in the DNS \"domain\" name
+--   specification. In RFC1034, it is acknowledged that a domain name
+--   input with a trailing \".\" will represent an absolute domain
+--   name (i.e. with respect to the DNS root). However, the grammar in
+--   RFC1035 disallows a trailing dot.
+--
+--   This makes some sense: within the DNS, everything knows its
+--   position in the tree. The relative/absolute distinction only
+--   makes sense on the client side, where a user's resolver might
+--   decide to append some suffix to a relative
+--   request. Unfortunately, that's where we live. So we have to deal
+--   with the possibility of having a trailing dot at the end of any
+--   domain name.
+--
+data UserDomain =
+  UserDomainRelative Domain |
+  UserDomainAbsolute Domain
+  deriving (Eq, Show)
+
+instance Pretty UserDomain where
+  pretty_show (UserDomainRelative d) = pretty_show d
+  pretty_show (UserDomainAbsolute d) = (pretty_show d) ++ "."
+
+
+-- | Parse a 'UserDomain'. This is what we'll be using to read user
+--   input, since it supports both relative and absolute domain names
+--   (unlike the implicitly-absolute 'Domain').
+--
+--   ==== _Examples_
+--
+--   >>> import Text.Parsec ( parse, parseTest )
+--
+--   We can really parse the root now!
+--
+--   >>> parseTest user_domain "."
+--   UserDomainAbsolute DomainRoot
+--
+--   But multiple dots aren't (only the first):
+--
+--   >>> pretty_print $ parse user_domain "" ".."
+--   .
+--
+--   We can also optionally have a trailing dot at the end of a
+--   non-empty name:
+--
+--   >>>  pretty_print $ parse user_domain "" "www.example.com"
+--   www.example.com
+--
+--   >>>  pretty_print $ parse user_domain "" "www.example.com."
+--   www.example.com.
+--
+--   A \"relative root\" can also be parsed, letting the user's
+--   resolver deal with it:
+--
+--   >>> parseTest user_domain ""
+--   UserDomainRelative DomainRoot
+--
+user_domain :: Parser UserDomain
+user_domain = try absolute <|> relative
+  where
+    absolute :: Parser UserDomain
+    absolute = do
+      d <- domain
+      _ <- char '.'
+      return $ UserDomainAbsolute d
+
+    relative :: Parser UserDomain
+    relative = fmap UserDomainRelative domain