alternate strategy for Y1,Y2

Signed-off-by: Jim Pryor <profjim@jimpryor.net>

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+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
+