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Question on Positive[x]=True

Dear Steve,

I have the following question.  I want to state that a variable 
x is positive. On page 1170 of the Third edition of The Mathematica book 
it is said that I can do it by making a definition
    Positive[x] = True 

However, while trying to do it I get a message : 
Set::write: Tag Positive in Positive[x] is Protected 

Any suggestions on how one can make variables positive, negative etc? 

Thanks a lot. 
                  Raya Khanin. 
                  University of Edinburgh

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