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Bug in pattern parsing?

• To: mathgroup at smc.vnet.net
• Subject: [mg61996] Bug in pattern parsing?
• From: Kristjan Kannike <kkannike at physic.ut.ee>
• Date: Tue, 8 Nov 2005 03:41:36 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

Hello,

I may have discovered a bug in pattern parsing or applying transformation
rules.

The following two rules should be equivalent:

rule1 = c_.*X_.Y_ -> X.Y

with c an optional variable, and

rule2 = c_.*Dot[X_, Y_] -> X.Y

Yet applying rule1 on a.b as

a.b/.rule1

gives

1.a.b

(the same result obtains for the optional factor actually present as in
const a.b), but

a.b/.rule2

gives

a.b

as it should.

I think that it has to do with the dot in c_., but curiously I get the
same result when writing the LHS of rule1 as Optional[c]*Y.Z and that
confuses me...

Any thoughts?

Kristjan Kannike
<http://www.physic.ut.ee/~kkannike/english/>

• Follow-Ups:
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  • Re: Bug in pattern parsing?
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>