# File lib/cairo/paper.rb, line 72
      def resolve_unit(size, from_unit, to_unit)
        from_unit ||= default_unit
        return size if from_unit == to_unit
        from_units = @@unit_resolvers[from_unit] || []
        raise UnknownUnit.new(from_unit) if from_units.empty?
        from_units.each do |unit, resolver|
          return resolver.call(size) if to_unit == unit
        end
        raise UnknownUnit.new(to_unit)
      end