def map
      RelativeSet.new(@hash.keys.inject({}) do |hash, key|
        hash[yield(key)] = true
        hash
      end)
    end
def map
      RelativeSet.new(@hash.keys.inject({}) do |hash, key|
        hash[yield(key)] = true
        hash
      end)
    end
@group Set Operations

 @return [RelativeSet]