public static Collector<Tuple2<Double,Double>, ?, Optional<Double>> regrR2Double() {
        return filter(t -> t.v1 != null && t.v2 != null, regrR2Double(t -> t.v1, t -> t.v2));
    }
Get a {@link Collector} that calculates the <code>regrR2Double()</code> function.
jOOL-java-8/src/main/java/org/jooq/lambda/Agg.java