Checking `(pop == true)` is just a very verbose way of saying `pop` since there are no other values that are considered true. I would write this as

```julia
pop && biased && error("Only unbiased estimator of population acv is available")
```

But that's just my very terse one-line error checking style. I suspect that @johnmyleswhite would probably favor this:

```julia
if pop && biased
  error("Only unbiased estimator of population acv is available")
end
```