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MathGroup Archive 1992

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Re: Pattern matching question

  • To: mathgroup at yoda.physics.unc.edu
  • Subject: Re: Pattern matching question
  • From: Silvio Levy <levy at math.berkeley.edu>
  • Date: Thu, 30 Jan 92 09:39:29 PST

Dan Dill notices that NonCommutativeMultiply[a_, b__, c_] := ...
causes NonCommutativeMultiply[x,y,z,t] to match a->x, b->y, c->z**t.
This is because NCM has the flat attribute, so Mathematica feels free
to try to group the arguments any old way in order to obtain a match.
So the match above is legal, and happens to be the first one encountered.
Of course the match a->x, b->(y,z), c->t (the one he wants) would also
be legal.

To avoid the wrong match, say ClearAttributes[NCM,Flat].

Silvio





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