private static <T> double variance0(List<T> l, ToDoubleFunction<? super T> function) {
        double average = avg0(l, function);
        return avg0(l, t -> Math.pow(function.applyAsDouble(t) - average, 2));
    }
Get a {@link Collector} that calculates the <code>VAR_POP()</code> function.
jOOL/src/main/java/org/jooq/lambda/Agg.java