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Sparse Array Question


I have a list of rules for defining a SparseArray.  The list has some 
coordinates repeated with different values.  If a coordinate appears in 
the list more than once, I want to sum up the values, and then have a 

i.e., if my list were myList={{1,1}->1,{2,1}->2,{1,1}->3} then my 
SparseArray should have {1,1}->4.  I've tried using ReplacePart on an 
array to say myArray=ReplacePart[myArray,...] but this seems horribly 
inefficient as a way to change values in an array.

I'm using Mathematica 5.2



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