[en] The WRS Modular Programming System has been developed as a means by which programmes may be more efficiently constructed, maintained and modified. In this system a module is a self-contained unit typically composed of one or more Fortran routines, and a programme is constructed from a number of such modules. This report describes one WRS module, the function of which is to calculate the uncollided flux and first-collision source in a slab geometry system containing a source distributed through some or all of the slabs. The information given in this manual is of use both to the programmer wishing to incorporate the module in a programme and to the user of such a programme. (author)