function add_tag(txtstart,txtend) {
	window.opener.document.getElementById('editor_field').focus();
	if (window.opener.document.selection) {
			// ie & may be opera 8     
			var rng = window.opener.document.selection.createRange();
			if (rng.text) {
				window.opener.document.selection.createRange().text = txtstart + rng.text + txtend;
				} else {
				window.opener.document.getElementById('editor_field').value += txtstart  + txtend;
				}
			window.opener.document.getElementById('editor_field').focus();
			}
	else if (window.opener.document.getElementById('editor_field').selectionStart || window.opener.document.getElementById('editor_field').selectionStart == '0') {
	// mozilla: intellegent bcodes support     
	var selStart = window.opener.document.getElementById('editor_field').selectionStart;
	var selEnd = window.opener.document.getElementById('editor_field').selectionEnd; 
	var s = window.opener.document.getElementById('editor_field').value;     
	s = s.substring(0, selStart) + txtstart + s.substring(selStart, selEnd) + txtend + s.substring(selEnd, s.length);
		window.opener.document.getElementById('editor_field').value = s; 
	if (selEnd != selStart) {
		window.opener.document.getElementById('editor_field').selectionStart = selStart;
		window.opener.document.getElementById('editor_field').selectionEnd = selEnd + txtstart.length + txtend.length;
		} else {
		window.opener.document.getElementById('editor_field').selectionStart = selStart + txtstart.length;
		window.opener.document.getElementById('editor_field').selectionEnd = window.opener.document.getElementById('editor_field').selectionStart;
		}
	} else {
	// other browsers     
	window.opener.document.getElementById('editor_field').value += txtstart + txtend;   
	}
	}