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How to call an old build-in function in a new version of Mathematica?


In order to use the UnitStep of Mathematica 5.2, I have to install two different versions 5.2 and 11.3 at the same time because Unitstep is different in the two versions (UnitStep has different derivatives in the two different verions of Mathematica, I listed the difference here). Today I suddenly think of that: Can I call the old version UnitStep of MMA 5.2 in the new Mathematica 11.3? How to realize if it's true? Thank you.




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