sub getByKey {
    my $key = shift;
    my $value = $valByKey{ $key };
	my $idx = $idxByVal{ $value };
	push @keys, splice @keys, $idx, 1;
	
	## To maintain linkage, it becomes necessary 
	## to completely re-build %idxByVal
	
	%idxByValue = map{ $valByKey{ $key[ $_ ] } => $_ } 0 .. $#keys;
	
    return $value;
}