public static <T> Collector<T, ?, Optional<Double>> covarianceDouble(ToDoubleFunction<? super T> functionX, ToDoubleFunction<? super T> functionY) {
        return collectingAndThen(toList(), covarianceFinisher(functionX, functionY, Agg::avg0));
    }
Get a {@link Collector} that calculates the <code>COVAR_POP()</code> function.
jOOL-java-8/src/main/java/org/jooq/lambda/Agg.java