class Sass::Script::Number

def coercion_factor(from_units, to_units)
  # get a list of unmatched units
  from_units, to_units = sans_common_units(from_units, to_units)
  if from_units.size != to_units.size || !convertable?(from_units | to_units)
    raise Sass::UnitConversionError.new("Incompatible units: '#{from_units.join('*')}' and '#{to_units.join('*')}'.")
  end
  from_units.zip(to_units).inject(1) {|m,p| m * conversion_factor(p[0], p[1]) }
end