// **Exercise 15.1**: Add an operation `compare_and_inc(&self, test: usize, by: usize)` that increments the counter by
// `by` *only if* the current value is `test`.
//
// **Exercise 15.1**: Add an operation `compare_and_inc(&self, test: usize, by: usize)` that increments the counter by
// `by` *only if* the current value is `test`.
//