Measurement.define(:mm) do |unit|
  unit.alias :millimeter, :millimeters
  unit.convert_to(:km) { |value| value / 1_000_000.0 }
  unit.convert_to(:hm) { |value| value / 100_000.0 }
  unit.convert_to(:dam) { |value| value / 10_000.0 }