// If backward_tape is not null, then this call to Accumulate is the result
    // of a still-active call to Accumulate which is running operations. We
    // forward these operations to backward_tape so the outer Accumulate call
    // can do its work.
    //
    // Rather than re-entering and delegating Accumulate like this, we could
    // instead allow ForwardAccumulator some control over the current tape set