```ruby
require 'rexml/document'

module Resync
  module XML
    class Parser

      # ------------------------------
      # Public methods

      # @return [Urlset, Sitemapindex]
      def self.parse(xml)
        root = find_root(xml)
        parse_class = get_parse_class(root)
        parse_class.load_from_xml(root)
      end

      # ------------------------------
      # Private methods

      def self.find_root(xml)
        case xml
        when String
          REXML::Document.new(xml).root
        when REXML::Document
          xml.root
        when REXML::Element
          xml
        else
          fail "Unexpected argument type; expected XML document, was #{xml.class}"
        end
      end

      def self.get_parse_class(root)
        root_name = root.name
        case root_name.downcase
        when 'urlset'
          Urlset
        when 'sitemapindex'
          Sitemapindex
        else
          fail "Unexpected root tag <#{root_name}>"
        end
      end

      private_class_method :get_parse_class

    end
  end
end
```

Change Parser from class to module
user
```ruby
require 'rexml/document'

module Resync
  module XML
    module Parser

      # @return [Urlset, Sitemapindex]
      def self.parse(xml)
        root = find_root(xml)
        parse_class = get_parse_class(root)
        parse_class.load_from_xml(root)
      end

      # ------------------------------
      # Private methods

      def self.find_root(xml)
        case xml
        when String
          REXML::Document.new(xml).root
        when REXML::Document
          xml.root
        when REXML::Element
          xml
        else
          fail "Unexpected argument type; expected XML document, was #{xml.class}"
        end
      end

      private_class_method :find_root

      def self.get_parse_class(root)
        root_name = root.name
        case root_name.downcase
        when 'urlset'
          Urlset
        when 'sitemapindex'
          Sitemapindex
        else
          fail "Unexpected root tag <#{root_name}>"
        end
      end

      private_class_method :get_parse_class

    end
  end
end
```
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