[phenixbb] Select atoms by B-factors?

Hi, I'm currently refining a structure at 1.6A. Substantial portions of the structure are very well ordered, with B-factors < 15. I'd like to refine these well ordered regions using anisotropic thermal parameters, while keeping everything else isotropic. However, in doing so, the atom selections tend to become quite convoluted. Is there any way to select all atoms which initially have an isotropic B less than a certain cuttoff value? That way one could (after isotropic B-factor refinement) select the most ordered regions of the structure for anisotropic refinement. Cheers, Oliver Clarke