public static <T> Collector<T, ?, Optional<Double>> regrSxyDouble(ToDoubleFunction<? super T> functionX, ToDoubleFunction<? super T> functionY) {
        // REGR_SXY() is like COVAR_POP(), but using SUM() instead of AVG() at the end
        return collectingAndThen(toList(), covarianceFinisher(functionX, functionY, Agg::sum0));
    }
Get a {@link Collector} that calculates the <code>REGR_SXY()</code> function.
jOOL-java-8/src/main/java/org/jooq/lambda/Agg.java