adding new functionality to a function non-intrusively!

[mjs7231]

Whenever I want to add functionality to a function while still allowing
it to word as it was before my edit would be to include a few optional
variables passed to the string.  something to this effect would look
like:

------------------------------------------
BEFORE:
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def myfunction(a, b):
    return (a + b)
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AFTER:
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def myfunction(a, b, op="add"):
    if (op == "add"):
        return (a + b)
    else:
        return (a - b)
------------------------------------------