public static AggregateOperation1<CharSequence, StringBuilder, String> concatenating() {
        return AggregateOperation
                .withCreate(StringBuilder::new)
                .<CharSequence>andAccumulate(StringBuilder::append)
                .andCombine(StringBuilder::append)
                .andExportFinish(StringBuilder::toString);
    }
@Test
    public void when_concatenating_withDelimiter() {
        validateOpWithoutDeduct(
                concatenating(","),
                StringBuilder::toString,
                "A",
                "B",
                "A",
                "A,B",
                "A,B"
        );
    }