... Next: An Abstract Data Up: Data Models and Previous: What is an . ... We describe a possible data structure designed to improve propagation of physics information in an analysis system, and an associated subroutine library for combining physical units. ... It has the size and shape of the physical data array, and its pixel values represent the exposure time for that element of the array. ... A Unit handling library parses physical unit definitions, and provides simple routines to combine them. ...