public static <T> Collector<T, ?, Optional<Double>> regrSyyDouble(ToDoubleFunction<? super T> functionX, ToDoubleFunction<? super T> functionY) {
        return collectingAndThen(
            collectors(
                regrCount(functionX, functionY),
                varianceDouble(functionX)
            ),
            t -> t.v2.map(v2 -> t.v1 * v2)
        );
    }
Get a {@link Collector} that calculates the <code>REGR_SYY()</code> function.
jOOL-java-8/src/main/java/org/jooq/lambda/Agg.java