= L (g (t**x) y) -- uses t, wants it as the left arg
| R (g (x**t) y) -- uses t, wants it as the right arg
| B (g (t**x) y) (g (x**t) y) -- uses t, doesn't care which arg
= L (g (t**x) y) -- uses t, wants it as the left arg
| R (g (x**t) y) -- uses t, wants it as the right arg
| B (g (t**x) y) (g (x**t) y) -- uses t, doesn't care which arg