Page 124

Location: Chapter 12, Exercise 5(a) at end of chapter

It is (requirement on the bird)

\Gamma xyzwv = y(zw)(xywv)

It should become

\Gamma xyzwv = y(zw)(xyzwv)

Short explanation

The need for the missing variable in the definition can be confirmed by plugging in the solution for \Gamma given a few pages later.