Description
Fetch-and-add function.Arguments
fav
Fetch-and-add variable (Rdsm integer vector of 
length 1), name quoted. Details
When a client calls fa() on fav, the quantity
inc (which could be negative) will be added to fav,
atomically, i.e. without interference by another thread.  The actual
addition will be performed at the server, thus reducing communications
costs.