Small things...

Good grief... I've let this go for a month. (UPDATE: No, I haven't. It helps to read the dates carefully.)

An option in pattern matching that I wasn't fully aware of: you can use _ when you don't care about what is there, and have no need to refer to it. I guess I did use it on the ByteString readInteger that returns Maybe(Integer, ByteString):

process line = case map B.readInteger (B.words line) of
               [Just (m, _), Just (n, _)] -> numXs m n
               _                          -> -1

But I didn't realize that I could use it in

ysInRange m n d
    | nVal == n = nPos - mPos + 1
    | otherwise = nPos - mPos
    where (_,    mPos) = findFirst' m
          (nVal, nPos) = findFirst' n
          findFirst' x = case findFirst (Ge x, Any) (dDigitYTree d) of
                             Just (Max i, Max j) -> (i, j)
                             Nothing             -> (tenToThe d, numNDigitYs d)

because we need only check for an exact match at the high end.