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Re: Why is NonCommutativeMultiply Protected?

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  • Subject: [mg82726] Re: Why is NonCommutativeMultiply Protected?
  • From: David Bailey <dave at>
  • Date: Tue, 30 Oct 2007 03:21:28 -0500 (EST)
  • References: <fg1ivs$k16$>

Szabolcs Horvát wrote:
> Why is NonCommutativeMultiply Protected?
> Is it safe to redefine NonCommutativeMultiply?  Does it have any special 
> built-in properties (apart from being Flat and OneIdentity)?  Is it used 
> by any of the built-in functions?
> NonCommutativeMultiply certainly does not evaluate to anything, but the 
> same can be said about Rule[], which is not at all safe to redefine 
> because: 1. It has some special properties (it affects scoping, it does 
> not like Unevaluated[], etc.)  2. It is used together with functions 
> such as Replace, and (most probably) built-in functions use it internally.
> So is it safe to Unprotect[NonCommutativeMultiply] and attach 
> definitions to it?  (I know that in certain situations one can use 
> UpValues, but I would like to know the answer to this question.)
Yes, NonCommutativeMultiply is meant for user use, so go ahead and 
Unprotect it. It is, however, extremely ugly, and might get confused 
with the Fortran exponentiation operator! I prefer to use CircleTimes, 
or another built-in operator and give it the same attributes as 

David Bailey

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