dropSpaces t = case BS.dropWhile isSpace t of
  t' | "--" `BS.isPrefixOf` t' -> dropSpaces $ dropRestOfLine t'
     | otherwise -> t'
dropSpaces t = case BS.dropWhile isSpace t of
  t' | "--" `BS.isPrefixOf` t' -> dropSpaces $ dropRestOfLine t'
     | otherwise -> t'
dropSpaces t = case BS.dropWhile isSpace t of
  t' | "--" `BS.isPrefixOf` t' -> dropSpaces $ dropRestOfLine t'
     | otherwise -> t'