[Discuss-gnuradio] forecast and general work function

I am trying to create a block that has two input and one output. Its like a priority multiplexer; if input0 is empty then output = input1 else output = input0 ( input0 always has samples) its a stream based block.
How do I declare the work function, because the number of samples needed from each input to produce noutput_items depends on whether input0 is empty.
Any help in the general work function is also greatly appreciated.

Thanks
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