When AutoOpts generates the code to parse the command line options, it has
the ability to produce any of several types of main() procedures.
This is done by specifying a global structured value for
main. The values that it contains are dependent on the value set for
the one value it must have: main-type.
The recognized values for main-type are guile,
shell-process, shell-parser, main, include,
invoke, and for-each.