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Trivial changes to formula

• To: mathgroup at smc.vnet.net
• Subject: [mg32533] Trivial changes to formula
• From: stevebg at adelphia.net (Steve Gray)
• Date: Sat, 26 Jan 2002 04:07:50 -0500 (EST)
• Organization: The Math Forum
• Sender: owner-wri-mathgroup at wolfram.com

  I have a large series of expressions like this:
((-z0+za)(-z0+zb)(-z0+zc))/((-z0+z1)(-z0+z2)(-zo+z3))
where it is important, for recognizing a pattern that
may or may not run through the entire formula, to
change the order of some of the terms to (za-z0) 
(no sign change) or (z0-z1) (has a sign change), etc.
  I need to tell it conveniently which terms to reverse,
and I want it to automatically change the sign of the
whole expression if necessary to keep the value
correct.
  Is there a way to do this?