Of the problem domain elements, the modules representing Data are the most fundamental. The Fit and Plotting modules are somewhat higher level in the sense that Data and Model do not depend on them (The Response classes
currently have a weak dependence on Model through the need to represent convolution operations). More importantly, perhaps, is the fact that none of the models depend at all on XSPEC's command language or user interface, so
they can in principle be reused by another program that needs, say similar data representations.