Hi Anthony,

I am just playing around: If you actually want your problem solved by a rule then Bills solution is perfect! My remark here is that the same can be accomplished by proper definition of your function Xpee:

(* definition for symbolic arguments: *)
Xpee /: Xpee[a_] + Xpee[-a_] := Ziplt[a]
Xpee /: Xpee[a_] - Xpee[-a_] := Ziplt[a]
Xpee /: -Xpee[a_] + Xpee[-a_] := Ziplt[-a]
(* definition for numeric arguments: *)
Xpee /: Xpee[a_?Positive] + Xpee[b_?Negative] := Ziplt[a] /; a == -b
Xpee /: Xpee[a_?Positive] - Xpee[b_?Negative] := Ziplt[a] /; a == -b
Xpee /: -Xpee[a_?Positive] + Xpee[b_?Negative] := Ziplt[-a] /; a == -b

(This should be checked carefully!)

Note: What I get here is e.g.:

Xpee[-2] - Xpee[2]
Out :   Ziplt[-2]

which is in disagreement with Bills outcome! You write:

... a rule that combines "Xpee[aa] (+/-) Xpee[-aa]" to give ZipIt[aa]. For example Xpee[-1/5] - Xpee[1/5] yields ZipIt[1/5].

Isn't this a contradiction, i.e. shouldn't this give Xpee[-1/5] or -Xpee[1/5]?

Regards -- Henrik

Test this endlessly before you trust it

In[1]:= xprule = {Xpee[a_] + Xpee[b_] | Xpee[a_] - Xpee[b_] /; b == -a :> ZipIt[b]};
{Xpee[2] + Xpee[-2], Xpee[-2] + Xpee[2], Xpee[-x] + Xpee[x], 
 Xpee[2] - Xpee[-2], Xpee[-2] - Xpee[2], Xpee[-x] - Xpee[x], 
 Xpee[-x] + Xpee[y], Xpee[-x] - Xpee[y]} /. xprule

Out[2]= {ZipIt[2], ZipIt[2], ZipIt[x],
ZipIt[-2], ZipIt[2], ZipIt[x], 
Xpee[-x] + Xpee[y], Xpee[-x] - Xpee[y]}