class SassC::Script::Value::Number

def sans_common_units(units1, units2)
  units2 = units2.dup
  # Can't just use -, because we want px*px to coerce properly to px*mm
  units1 = units1.map do |u|
    j = units2.index(u)
    next u unless j
    units2.delete_at(j)
    nil
  end
  units1.compact!
  return units1, units2
end