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Custom Format for (b-a)


Hello Math Group,

When Mma displays an expression involving sums the terms are
always sorted.  Consider the following example:
Exp[c+w (z-r)]+a(z-r)

This is displayed as:
Exp[c+w(-r+z)]+a(-r+z)

It comes out this way because Sort[{z, -r}]->{-r,z}.
I believe the output looks better if "z-r" is used rather than "-r+z".
I would only want it this way when two terms are added, and I would want it 
to apply
regardelss of where the sum is in the expression.

I thnik this can be done using the following:

Format[Plus[Times[-1,a_], b_]]:=  ..........some output............... 
/;OrederedQ[a,b]

However, I haven't been able to come up with code that gives adequate 
results.
Can anyone help?
               T.E.



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