A mosaic detector has a ``problem of numbers'' which results from the large number of identical and permutable hardware elements involved in its construction. In order to describe a mosaic, and the data produced by a mosaic, a great deal of bookkeeping information is required. In the case of the physical hardware, coaxial cables carrying video signals from one point to another can easily be permuted. It is important that the software can be reconfigured just as easily so that the resulting images will be properly documented.
The bookkeeping information for mosaics from UCO/Lick Observatory is stored in C data structures which describe mosaic layout, wiring, and configuration. At run time the data acquisition program is instructed to select one of the data structures. The data in the selected structure are inserted into the resulting FITS files in order to document which image pixels came from where, and how they go there.
The data structures which describe the mosaics were originally modelled in XML. The C data structures are isomorphic to the XML. Currently the translation between C and XML is being done manually. Eventually we hope to compile directly from the XML into C.