+ 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. (\f x. A) (u g)) in
+ let g = Y ( \g. (\f y. B) (u g)) in
+ let f = u g in