be more specific with equivalences relating apd_f and ap_f

This might be nitpicking, but the former wording is incorrect: For (2.3.7), one has to concatenate with the inverse of transportconst^B_p(f(x)). I can't see how this path could be read as transportconst^B_p(b) for a suitable b. 
On the other hand, the change makes Exercise 2.5 an instance of Ex. 2.6.