other helper: accumulate the Pos pos found so far in the first param

-- |
-- >>> foo [] "hi\n  " (Pos 4 16)
-- [Pos 4 16,Pos 4 17,Pos 4 18,Pos 5 0,Pos 5 1]

foo :: [Pos] -> String -> Pos -> [Pos]
foo found []       _      = found
foo 
foo :: [Pos] -> String -> Pos -> [Pos]
foo found []       _      = found
foo found []       _      = found