function switch_category(category){
  if(category=='all'){
    reset_category_links();
    $$('.listings tbody tr').each(
      function(value, index){value.show();}
    );
    $('all_cat').className = 'cat active';
  }
  else{
    reset_category_links();
    $$('.listings tbody tr').each(
      function(value, index){value.hide();}
    );
    $$('.listings tbody tr.'+category+'_job').each(
      function(value, index){value.show();}
    );
    $(category+'_cat').className = 'cat active';
  }
}

function reset_category_links(){  
  $$('.cat').each(
    function(value, index){
      value.className = 'cat';
    }
  );
}