android/guava/src/com/google/common/collect/RegularImmutableTable.java
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);
  }
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);
  }
/*
We could have declared this method 'static' but the additional compile-time checks achieved by
referencing the type variables seem worthwhile.