st: How can I check that a variable exists?

["Benjamin Villena Roldan" <bvillena@troi.cc.rochester.edu>]

Dear statalist,
This may sound like a trivial question, but I unsuccessfully looked for an
answer so far.
  I need to perform certain action A if there is a variable XA defined in
the dta file, and an action B if there is a variable XB in the dta file. How
can I do so? Any ideas?

Thanks

Benjamin


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