Accept an underscore as a label prefix in the modern domain parser.

src/GenericParser/DomainParser.purs

@@ -26,13 +26,14 @@ import GenericParser.Parser (Parser(..)
 label :: Parser DomainError String
 label = do
   pos <- current_position
+  maybeu <- tryMaybe $ char '_'
   maybel <- tryMaybe letter
   case maybel of
     Nothing -> Parser \_ -> failureError pos (Just InvalidCharacter)
     Just l -> do
       s <- tryMaybe ldh_str
       lastpos <- current_position
-      let labelstr = CU.singleton l <> maybe "" CU.fromCharArray s
+      let labelstr = maybe "" (\_ -> "_") maybeu <> CU.singleton l <> maybe "" CU.fromCharArray s
       if (S.length labelstr > max_label_length)
       then Parser \_ -> failureError pos (Just <<< LabelTooLarge $ S.length labelstr)
       else if (S.length labelstr > 1 && not (parse_last_char labelstr let_dig))

src/GenericParser/Parser.purs

@@ -121,8 +121,10 @@ try p = Parser p'
                      _       -> failureError position error
 
 sat :: forall e. (Char -> Boolean) -> Parser e Char
-sat p = do x <- item
-           if p x then pure x else empty
+sat p = do
+           pos <- current_position
+           x <- item
+           if p x then pure x else Parser \input -> failure pos
 
 digit :: forall e. Parser e Char
 digit = sat isDigit