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Re: Positive[a] = True ???

  • Subject: [mg2530] Re: [mg2518] Positive[a] = True ???
  • From: hay at (Allan Hayes)
  • Date: Thu, 16 Nov 1995 06:46:28 GMT
  • Approved:
  • Distribution: local
  • Newsgroups: wri.mathgroup
  • Organization: Wolfram Research, Inc.
  • Sender: daemon at ( )

Frank Domokos <domokos at>
in [mg2518] Positive[a] = True ???
asks about the following kind of response

IN>> Positive[a] = True;
OUT>> Set::write: Tag Positive in Positive[a] is Protected.


Most system functions, like Positive, are protected (have the  
attribute Protected) to avoid unintentional changes. You cannot make  
a definition for them ("tagged" by them) without unprotecting them  
(see later).

Here are some ways round this for your example

1.Tag your definition with a instead of f:

   a. by using UpSet (^=) instead of Set (=):

      Positive[a]^= True;

   or, more specifically,

   b. by using TagSet:

      a/:Positive[a] = True;


2. Unprotect Positive to allow definitions to be tagged by it:


   Positive[a] = True;

 (eventually re-protecting Positive)

Of course if a is protected or is an expression with a protected  
head then you will need to unprotect either Positive or a or the  
head of a.

Allan Hayes
hay at

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