Document maybeLastIndex.

src/Range.hs

@@ -34,6 +34,7 @@ rangesAndWordsFrom startIndex string = maybe [] id $ takeAndContinue <$> (word <
     -- | > A member of one of the following Unicode general category _Letter_, _Mark_, _Number_, _Connector_Punctuation_
     isWord c = Char.isLetter c || Char.isNumber c || Char.isMark c || Char.generalCategory c == Char.ConnectorPunctuation
 
+-- | Return Just the last index from a non-empty range, or if the range is empty, Nothing.
 maybeLastIndex :: Range -> Maybe Int
 maybeLastIndex (Range start end) | start == end = Nothing
 maybeLastIndex (Range _ end) = Just $ end - 1