** Recursive Cases: Operators**

The recursive case has a ` rhs` that is an operation:

` (= ` *α* ` (` *op β γ* ` ))`

We are hoping that the desired variable will be somewhere in
*β* or *γ*; to get to it, we must apply some kind of
inverse operation to both sides of the equation to get rid of *op*
and isolate *β* or *γ*.

In general, there may be two inverse operations to try.

We can produce the result of the inverse operation by constructing
a new equation from the given one, e.g., given:

` (= ` *α* ` (+ ` *β γ* ` ))`

we can construct two new possibilities:

` (= (- ` *α β* ` )` *γ* ` )`
(subtract *β* from both sides)

` (= (- ` *α γ* ` )` *β* ` )`
(subtract *γ* from both sides)

After making a new equation, we simply call ` solve` to try
to solve * that* equation. We return the first solution
that is not ` null`.