module Sass::Value::Number::Unit

def canonicalize_units(units)
  return units if units.empty?
  if units.length == 1
    type = TYPES_BY_UNIT[units.first]
    return type.nil? ? units : [UNITS_BY_TYPE[type].first]
  end
  units.map do |unit|
    type = TYPES_BY_UNIT[unit]
    type.nil? ? units : [UNITS_BY_TYPE[type].first]
  end.sort
end