Can somebody check that I've implemented this correctly? *****X*X*X*XX***X*X*XXX*X*X*XX***X*X*X*XX**X*X*XX**X*X*X*XX ***X*X*XXX*X*X*XX**XX*X*XX***X*X*XXX*X*XX****X*X*X*XX****X* X*X*XX***X*X*XXX*X*X*XXX*X*XXXX****X*X*XXX****X*X*X*XX***X* X*XXX*X*X*XXX*X*XXXX A parser for this is given by decode (c:cs) = case c of 'X' -> X '*' -> let (e0,cs0) = decode cs; (e1,cs1) = decode cs0 in (e0 `apply` e1, cs1) The letter X stands for the following combinator: X = \x -> xSK K = \xy -> x S = \fgx -> fx(gx) Assuming my Haskell code is correct, the above *should* be a program to compute the sum of the Church numberal 2 and the Church numeral 2. In other words, this is the world's most verbose way to define the act of putting two and two together. ;-) But... I don't know... that's A LOT of expression there! Have a slipped up somewhere? Does anybody have a way to check?