code: function excludes(...opts){
    return function exclusionCheck(argv, options){
      let n = _(opts)
  	.map(_.propertyOf(argv))
  	.filter()
  	.size();
      if(n > 1){
        let displayOpts = _.map(opts, (o) => '"-' + o + '"');
        let errorMsg = 'More than one mutually exclusive option'
  	  + ' in the set of [' + displayOpts + ']'
  	  + ' has been specified.'
        throw new Error(errorMsg);
      }
      return true;
    }
  }
docs: /* ARGV checks