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.