code: public Option defaultValue(String defaultValue) throws RequiredParametersException {
  		if (this.choices.isEmpty()) {
  			return this.setDefaultValue(defaultValue);
  		} else {
  			if (this.choices.contains(defaultValue)) {
  				return this.setDefaultValue(defaultValue);
  			} else {
  				throw new RequiredParametersException("Default value " + defaultValue +
  						" is not in the list of valid values for option " + this.longName);
  			}
  		}
  	}
docs: Define a default value for the option.
  
  @param defaultValue - the default value
  @return the updated Option
  @throws RequiredParametersException if the list of possible values for the parameter is not empty and the default
  value passed is not in the list.