MODSUM Program MODSUM sums one or more VLBI models into a single model, and can shift or scale these models. To run the program, type command MODSUM (defined in the VLB:LOGIN.COM file). It will ask you to supply answers to the following questions: 1. Input model file? 2. Factor by which to multiply fluxes in this file? Questions (1) and (2) are repeated until (1) is answered with a . 3. Output model file? 4. Shift models so that 1st component is at origin? The program will accept from 1 to 10 different model files. While there is no limit on the final number of components, some programs cannot handle more than 2000. Most questions about how to run this program can be answered when you actually run the program. This is a very simple minded program. It can be used to add two or more models (make sure the factors add up to 1.0); it can be used to shift a single model around; it can be used to subtract two models (let the factors be +1 and -1). R.S. Simon, 1981 September 7