Note the asymmetry in the Add and Done methods. Add, which increments the counter, must be called before the worker goroutine starts, not within it; otherwise we would not be sure that the Add happens before the “closer” goroutine calls Wait. Also, Add takes a parameter, but Done does not; it’s equivalent to Add(-1). We use defer to ensure that the counter is decremented even in the error case. The structure of the code above is a common and idiomatic pattern for looping in parallel when we don’t know the number of iterations.
