be more specific in the definition of the equivalences relating ap_f and apd_f

This might be nitpicking, but the current wording here is incorrect: For 2.3.7 one has to concatenate with
the *inverse* of transconst B p {f(x)}. I don't see how this could be an instance of transconst B p b  with a 
suitable b.

The change would make Exercise 2.5 a special case of Ex. 2.6.