public static <T> Collector<T, ?, Long> regrCount(ToDoubleFunction<? super T> functionX, ToDoubleFunction<? super T> functionY) {
        return count();
    }
Get a {@link Collector} that calculates the <code>REGR_COUNT()</code> function.
jOOL/src/main/java/org/jooq/lambda/Agg.java