Example

Suppose the user has his additive model routine in the file
USRDISK:[RONTGEN.XSPEC]USER1.FOR.

$ set def usrdisk:[rontgen.xspec]     | Change directory.

$ fortran user1                       | Compile.

$ mkumd user1                         | Link the executable.

$ setumd user1                        | Set logical umdshr.

$ xspec                               | Start xspec.

XSPEC> model wabs useradd1            | Define model.