class Sass::Script::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
  return units1.map do |u|
    next u unless j = units2.index(u)
    units2.delete_at(j)
    nil
  end.compact, units2
end