# File lib/facets/core/module/sort_on.rb, line 19
  def sort_on(*fields)
    code = %{def <=>(o)\n}
    fields.each { |f|
      code << %{cmp = ( @#{f} <=> o.instance_variable_get('@#{f}') );
                return cmp unless cmp == 0\n}
    }
    code << %{0\nend; alias_method :cmp, :<=>;}
    class_eval( code )
    fields
  end