gistfile1.js

Components.utils.import("resource:///modules/devtools/gcli.jsm");

gcli.addCommand({
  name: 'foo',
  params: [
    {
      name: 'letters',
      type: {
        name: 'selection',
        data: function() {
          return [ 'aaaa', 'bbbb', 'cccc' ];
        }
      }
    }
  ],
  exec: function(args, context) {
    return args.letters;
  },
});