code: function enumerable(rule, value, source, errors, options) {
    rule[ENUM] = Array.isArray(rule[ENUM]) ? rule[ENUM] : [];
    if (rule[ENUM].indexOf(value) === -1) {
      errors.push(util.format(options.messages[ENUM], rule.fullField, rule[ENUM].join(', ')));
    }
  }
docs: Rule for validating a value exists in an enumerable list.
  
  @param rule The validation rule.
  @param value The value of the field on the source object.
  @param source The source object being validated.
  @param errors An array of errors that this rule may add
  validation errors to.
  @param options The validation options.
  @param options.messages The validation messages.