code: public CmdLineAction arg(String arg) {
  		if (!CmdLineProcessor.isActionArg(arg)) {
  			throw new IllegalArgumentException(arg);
  		}
  		this.args.add(arg);
  		return this;
  	}
docs: Add an argument string that should be trigger this {@linkplain CmdLineAction}.
  
  @param arg The argument string to be associated with this {@linkplain CmdLineAction}.
  @return The updated {@linkplain CmdLineAction}.