collapse to unique row.names using mean()

["Phillip Staford" <biomining@hotmail.com>]

Dear S group;

I'm looking for a very low-memory (slow is OK, but memory is critical here) 
method to collapse a data.frame that has a mixture of unique and replicated 
row.names.  What's the best way to do this without burning unnecessary 
memory?
A  3
A  2
A  1
B  3
C  2
C  4
into
A  2
B  3
C  3


Thanks for any suggestions, the obvious ways are quick but not memory 
efficient I think.
Phillip

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