electrons are effectively excluded from this site. There have been several theoretical approaches to understand this apparent paradox. The first is specific to the STM method and relies on the fact that the impurity states are localized in the CuO$_2$ plane, two layers below the BiO surface probed by the STM tip; the intervening layers are then argued to provide a blocking layer-specific tunneling path which samples not the Cu directly below the tip, but preferentially the four nearest neighbors.[@IMartin:2002; @JXZhu:2000b] Some indirect support for this point of view has been provided by density functional theory,[@LLWang:2005] which finds that the pattern of LDOS near the impurity but close to the BiO surface can be quite different than the LDOS in the CuO$_2$ plane. On the other hand, this calculation, applicable only to the normal state, suggested that the hybridization of the wavefunctions involved is not only blocking layer-specific, but also specific to the particular chemical impurity in question. A second class of approaches obtains LDOS patterns similar to experiment for both Zn and Ni by simply assuming an [*ad hoc*]{} distribution of site potentials and nearby hoppings to tune the weights of on-site and nearest-neighbor LDOS.[@tangflatte; @JMTang:2004] While the bare 