I have used \newtheorem to define a class. I want to modify a previous instance of the class to present a slight variation. (A similar, but different, example is using a previous equation to number the current one.) Here is an example of the output I would like:

Assumption 1. An assumption.

Assumption 1'. A similar assumption.

I have tried using \tag with no success. Here is an example of my failed attempts:





    An assumption.

   A similar assumption.


My main interest is not in getting \tag to work, but in finding some (preferably simple) way to get numbering like this. Any help would be appreciated.



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