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<input type="text" name="key1" maxlength="6" size="6" onkeyup="CodeNext(1);">
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<input type="text" name="key2" maxlength="4" size="4" onkeyup="CodeNext(2);">
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<input type="text" name="key3" maxlength="5" size="5" onkeyup="CodeNext(3);">

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<script language="JavaScript" type="text/javascript">
function CodeNext (num) {
	var event = window.event;
	var code = event.which;
	if (num == 1) {
		if (document.forms[0].key1.value.length == 6) {
			if ((code != 37) && (code != 39)) {
				document.forms[0].key2.focus();
			}
			else if ((code == 39) && (document.forms[0].key1.selectionStart == 6) && (document.forms[0].key1.selectionEnd == 6)) {
				document.forms[0].key2.focus();
			}
		}
	}
	else if (num == 2) {
		if (document.forms[0].key2.value.length == 4) {
			if ((code != 37) && (code != 39)) {
				document.forms[0].key3.focus();
			}
			else if ((code == 39) && (document.forms[0].key2.selectionStart == 4) && (document.forms[0].key2.selectionEnd == 4)) {
				document.forms[0].key3.focus();
			}
		}
		else if ((document.forms[0].key2.selectionStart == 0) && (document.forms[0].key2.selectionEnd == 0)) {
			if (code == 37) {
				document.forms[0].key1.focus();
			}
		}
	}
	else {
		if (document.forms[0].key3.selectionStart == document.forms[0].key3.selectionEnd) {
			if (document.forms[0].key3.selectionStart == 0) {
				if (code == 37) {
					document.forms[0].key2.focus();
				}
			}
		}
	}
}
</script>