# File lib/cft/rpm.rb, line 46
    def self.diff(before, after)
        erased, installed, updated  = {}, {}, {}
        (before.keys + after.keys).uniq.each do |na|
            if ! before.key?(na)
                installed[na] = after[na]
            elsif ! after.key?(na)
                erased[na] = before[na]
            else
                upd = setdiff(after[na], before[na])
                updated[na] = upd unless upd.empty?
            end
        end
        return { :erased => erased, :installed => installed,
            :updated => updated }
    end