can I undefine a latex command?

[Guenter Milde]

I wonder if there is some TeX magic, that lets me undefine a LaTeX command,
  \Re 
say.

Then I could write a math_macro that has also the right look opposed to the
present version with the präamble renewcommand:
  \renewcommand{\Re}{\mathrm{Re}}

(of course, a macro feature that lets the user choice between \newcommand
  \providecommand (or no latex command) and \renewcommand is still on my wishlist)

GM

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