function addDoc(id){
    if(document.getElementById("fa_bestellen_cbx"+id).checked){
        document.getElementById("fa_ordered_docs").innerHTML=document.getElementById("fa_ordered_docs").innerHTML + "<div class='fa_formintro_ordered_doc' id='fa_formintro_ordered_doc" + id + "'>" + document.getElementById("fa_doctitle"+id).innerHTML + "</div>";
    }else{
        document.getElementById("fa_ordered_docs").removeChild(document.getElementById("fa_formintro_ordered_doc"+id));
    }
    
    if(document.getElementById("fa_ordered_docs").getElementsByTagName("div").length==1){
        document.getElementById("fa_nodocs_alert").style.display="block";
    }else{
        document.getElementById("fa_nodocs_alert").style.display="none";
    }
    //write documents into hidden field
    orderedDoksinput=document.doc_order_form.hid_ordered_docs;
    orderedDoksinput.value="";
    for(i=1;i<document.getElementById("fa_ordered_docs").getElementsByTagName("div").length;i++){
        orderedDok=document.getElementById("fa_ordered_docs").getElementsByTagName("div")[i].innerHTML;
        orderedDoksinput.value=orderedDoksinput.value+ " - " + orderedDok + "\n";
    }
}