code: final void checkNoDuplicate(R rowKey, C columnKey, V existingValue, V newValue) {
      checkArgument(
          existingValue == null,
          "Duplicate key: (row=%s, column=%s), values: [%s, %s].",
          rowKey,
          columnKey,
          newValue,
          existingValue);
    }
docs: /*
  We could have declared this method 'static' but the additional compile-time checks achieved by
  referencing the type variables seem worthwhile.