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###### borrowed code found online #################################################
#Description: Reverses the direction of a bidirectional mapping, removes an entries pointing to NULL
#Author: Matthew Young
#Date Modified: 13/12/2010
reverseMap=function(map){
tmp=unlist(map,use.names=FALSE)
names(tmp)=rep(names(map),times=as.numeric(summary(map)[,1]))
return(split(names(tmp),as.vector(tmp)))
}
#########################################
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