code: public static OpsGenieClientValidationException missingMultipleMandatoryProperty(String... propertyNames) {
          if (propertyNames != null && propertyNames.length > 0) {
              String missingProperties = "[" + propertyNames[0];
              for (int i = 1; i < propertyNames.length; i++)
                  missingProperties += " or " + propertyNames[i];
              missingProperties += "]";
              return new OpsGenieClientValidationException("Missing mandatory property " + missingProperties, MISSING_MANDATORY_PROPERTY);
          }
          return null;
      }
  }
docs: multiple miising property case!