function updateElementIndex(el, prefix, ndx) {
        var id_regex = new RegExp('(' + prefix + '-\\d+)');
        var replacement = prefix + '-' + ndx;
        el.id = el.id.replace(id_regex, replacement);
        el.name = el.name.replace(id_regex, replacement);
    }
    
    function addForm(btn, prefix) {
        var formCount = parseInt($('#id_' + prefix + '-TOTAL_FORMS').val());
        var row = $('.dynamic-form:first').clone(true).get(0);
        $(row).removeAttr('id').insertAfter($('.dynamic-form:last')).children('.hidden').removeClass('hidden');
        $(row).children().not(':last').children().each(function() {
    	    updateElementIndex(this, prefix, formCount);
    	    $(this).val('');
        });
        $(row).find('.delete-row').click(function() {
    	    deleteForm(this, prefix);
        });
        $('#id_' + prefix + '-TOTAL_FORMS').val(formCount + 1);
		register_keydown_handler();
        return false;
    }
    
    function deleteForm(btn, prefix) {
        $(btn).parents('.dynamic-form').remove();
        var forms = $('.dynamic-form');
        $('#id_' + prefix + '-TOTAL_FORMS').val(forms.length);
        for (var i=0, formCount=forms.length; i<formCount; i++) {
    	    $(forms.get(i)).children().not(':last').children().each(function() {
    	        updateElementIndex(this, prefix, i);
    	    });
        }
		berechne_brutto_netto();
        return false;
    }