Plese HELP finding center of surface

Hello everybody, I'm a novice user of AUTOCAD and I'm starting learning about AUTOLISP and Autocad customization...I'm lookin for an AUTOLISP
routine which can find the centre of a selected surface and possibly place a reference system, as much as possible (depending on the congiuration of the surface) parallel to the fixed one, in that point.
Will apreciate very much every kind of help
Alberto (preparing his mechanical engineering final thesis)
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