Appending

Hi,
I am using mathml as part of an application. I have two calculated fields x and y. I want to create a third field z which should be equal to x-y (x hyphen y) or x_y (x underscore y). Is this possible in mathml? My apologies if this is a very silly question. I am very new to this.

Any help is greatly appreciated.

Thanks a lot !!
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