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Adding options to built-in functions

• To: mathgroup at christensen.cybernetics.net
• Subject: [mg697] Adding options to built-in functions
• From: Jack Goldberg <jackgold at math.lsa.umich.edu>
• Date: Mon, 10 Apr 1995 13:05:27 -0400 (EDT)

Hi group:
	I would like to add an option to PowerExpand
called InverseTrig->False.  This option, when evoked 
would  reduce 
	ArcTan[Tan[x]]  to  x
and similarly for the other inverse trigs composed
with the trigs.  I have done this by defining a new 
function  InverseExpand.  I am quite pleased with 
my result but it strikes me that the proper place 
for this kind of manipulation is an option to 
PowerExpand since it is quite in keeping with the  
positive and negative features of this command.  
So let me repeat the question:

What is the preferred way to introduce your 
own option to a built in command?  Indeed, 
is it even possible? and if possible, desirable?
Jack