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 (`f'` is a variable not occurring free in `A`, `B`, or `C`):
let rec f' g x = (let f = f' g in A)
in let rec g y = (let f = f' g in B)
in let f = f' g in C
or, expanded into the form we've been working with:
let f' = Y (\f' g x. (\f. A) (f' g)) in
let g = Y (\g y. (\f. B) (f' g)) in
let f = f' g