Proportion of excess sequencing, for walking with two BAC libraries. The first library B has clones of insert size L = 1 and fixed depth d = 15, whereas the second library B′ has clones of insert size L′ = ½ and various depthsd′. Walking is performed as described in the text. The curves show the proportion of excess sequencing, as a function of mean initial ocean size θ and for various library depths (d = 10, 15, 25, 50, and ∞) for the second library B′.