public void addListener(Runnable listener, Executor executor) {
        CompletableFutures.allOf(operands)
                .whenCompleteAsync((__, ___) -> listener.run(), executor);
    }
<p>Adds a {@link CompletableFuture} to this conjunction. This can be called at any time, even after having called
{@link #get()} previously.</p>

@param operand The operand to add to this conjunction.
jdisc_core/src/main/java/com/yahoo/jdisc/handler/FutureConjunction.java