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pattern matching: rules that stop other rules?

• To: mathgroup at smc.vnet.net
• Subject: [mg71596] pattern matching: rules that stop other rules?
• From: croddie at princeton.edu
• Date: Fri, 24 Nov 2006 01:17:16 -0500 (EST)

Hello. I've been using Mathamatica for quite a while but without ever
finding out how the language works fundamentally, which I'm trying to
do now. I'd be grateful for some help in understanding patterns - it
seems like a powerful idea to me. There is something I can't work out.

Define a function f [x_]:=0 say
Now If [ p, 1, f [ 2] ] evaluates to itself. So the rule in the
definition is not applied to f [ 2 ].

Replace If with some other undefined function, say qwerty, and you get
qwerty [ p, 1, 0] not surprisingly. And If [ p, 1, f [ 2] ] /. f
[x_]->0 returns If [ p, 1, 0 ].

Is there a rule associated with If that stops a rule (if that's the
right expression) from being applied inside it? Can users write such
rules?

Thanks for any help.