Alternate strategy for Y1, Y2
* This is (in effect) the strategy used by OCaml. The mutually recursive:
let rec
f x = A ; A may refer to f or g
and
g y = B ; B may refer to f or g
in
C
is implemented using regular, non-mutual recursion, like this (`u` is a variable not occurring free in `A`, `B`, or `C`):
let rec u g x = (let f = u g in A)
in let rec g y = (let f = u g in B)
in let f = u g in C
or, expanded into the form we've been working with:
let u = Y (\u g x. (\f. A) (u g)) in
let g = Y (\g y. (\f. B) (u g)) in
let f = u g