docs: @Trivial
      private String validateNotification(Collection<?> added, Collection<?> removed, Collection<?> updated) {
          boolean isAddition = !added.isEmpty();
          boolean isRemoval = !removed.isEmpty();
          boolean isUpdate = !updated.isEmpty();
  
          if ( !isAddition && !isRemoval && !isUpdate ) {
              // Should never occur:
              // Completely null changes are detected and cause an early return
              // before reaching the validation method.
              return "null";
  
          } else if ( isAddition ) {
              return "Addition of [ " + added.toString() + " ]";
  
          } else if ( isUpdate && isRemoval ) {
              return "Update of [ " + updated.toString() + " ]" +
                     " with removal of [ " + removed.toString() + " ]";
  
          } else {
              return null;
          }
      }