How can I write custom crawler using PL?

The following code is an example of loading data via crawler with special function to generate link for downloading:

create procedure EUROPEANA_STORE (in _host varchar, in _url varchar, in _root varchar, inout _content varchar, in _s_etag varchar, in _c_type varchar, in store_flag int := 1, in udata any := null, in lev int := 0)
{
   declare url varchar;
   declare xt, xp any;
   declare old_mode int;
   xt := xtree_doc (_content, 2);
   xp := xpath_eval ('//table//tr/td/a[@href]/text()', xt, 0);
   commit work;
   old_mode := log_enable (3,1);
   foreach (any u in xp) do
     {
       u := cast (u as varchar);
       url := sprintf ('http://semanticweb.cs.vu.nl/europeana/api/export_graph?graph=%U&mimetype=default&format=turtle', u);
       dbg_printf ('%s', u);
	 {
	   declare continue handler for sqlstate '*' { 
	     dbg_printf ('ERROR: %s', __SQL_MESSAGE);
	   };
	   SPARQL LOAD ?:url into GRAPH ?:u;
	 }
     }
   log_enable (old_mode, 1);
   return WS.WS.LOCAL_STORE (_host, _url, _root, _content, _s_etag, _c_type, store_flag, 0);
}

How can I write custom crawler using PL?

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