Document spanThese.

src/Alignment.hs

@@ -153,6 +153,10 @@ group2 ranges children | Just (headRanges, tailRanges) <- unconsThese ranges
                        | ([]:rest) <- children = group2 ranges rest
                        | otherwise = ([] <$ ranges, children, fmap (flip (,) []) <$> sequenceL ranges)
 
+-- | Partitions and splits a list of children into a triple consisting of:
+-- | - elements which matched; if an element matches only partially this field will contain only the matching side
+-- | - the opposite sides of elements which matched only on the other side
+-- | - elements which do not intersect.
 spanThese :: (Join These a -> Join These Bool) -> [[Join These a]] -> ([[Join These a]], [[Join These a]], [[Join These a]])
 spanThese pred children | (child:rest) <- children
                         , not (null child)