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Re: Why doesn't this rule work?

• To: mathgroup at smc.vnet.net
• Subject: [mg32470] Re: Why doesn't this rule work?
• From: atelesforos at hotmail.com (Orestis Vantzos)
• Date: Tue, 22 Jan 2002 03:19:52 -0500 (EST)
• References: <a2ghu1$ki$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

The correct rule is 
Plus[n1_. a, n2_. c, R___] /; n1 == -n2 :> Plus[R]
Repeat after me: FullForm is my best friend, FullForm is my best
friend,...
Indeed FullForm[a-b-c] for instance returns
Plus[a,Times[-1,b],Times[-1,c]]
It is evident that a rule of the form (n1_. a + n2_. b) simply does
not match, since the only _Plus expression present contains many more
things than a and c.
Hope that helped,
Orestis