module Middleman::Sitemap::Extensions::Proxies::ResourceInstanceMethods

def get_source_file
  if proxy?
    proxy_resource = store.find_resource_by_path(proxied_to)
    unless proxy_resource
      raise "Path #{path} proxies to unknown file #{proxied_to}:#{store.resources.map(&:path)}"
    end
    if proxy_resource.proxy?
      raise "You can't proxy #{path} to #{proxied_to} which is itself a proxy."
    end
    proxy_resource.source_file
  end
end