public static <T, U> Collector<T, ?, Optional<Double>> stddevDouble(ToDoubleFunction<? super T> function) {
        return collectingAndThen(toList(), l -> l.isEmpty() ? Optional.empty() : Optional.of(Math.sqrt(variance0(l, function))));
    }
Get a {@link Collector} that calculates the <code>STDDEV_POP()</code> function.
jOOL/src/main/java/org/jooq/lambda/Agg.java