Page 131

Location: Chapter 13, discussion after Problem 8

It is (second paragraph)

In each of the problems 1, 2, 3, 4, we derived a sage bird from three proper combinators: one was irregular, the mockingbird, and the other two were regular.

It should become

In each of the problems 2, 3, 4, 6 we derived a sage bird from three proper combinators, one irregular (either $M$ or $Q$) and the other two regular.

Short explanation

In Problem 1 a Sage bird is constructed using the Mockingbird, the Robin and the Bluebird: of these, two are irregular ($M$ and $R$) and only one is regular, $B$. Hence, the problem list given in the text is to be revised.

One can include Problem 6, where a Sage bird is derived using the Queer bird (irregular), the Lark and the Warbler (both regular): but then, the explicit mention of the Mockingbird has to be expanded, since the irregular bird used here is another one.