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Re: How to evaluate parts of an expression, but not other parts?

• To: mathgroup at smc.vnet.net
• Subject: [mg122703] Re: How to evaluate parts of an expression, but not other parts?
• From: Rui <rui.rojo at gmail.com>
• Date: Mon, 7 Nov 2011 05:51:07 -0500 (EST)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <j930l9$a3d$1@smc.vnet.net>
• Reply-to: comp.soft-sys.math.mathematica at googlegroups.com

What about

ReleaseHold[
 Hold[(a + b) + (c + d)] /. 
  i_Symbol /; Context[i] === "System`" :> Hold[i]]

or maybe

ReleaseHold[
 Hold[(a + b) + (c + d)] /. 
  i_Symbol /; Context[i] =!= "Global`" :> Hold[i]]