Hi.
  How do I make an optional element to not exist if it has no inner elements?

For example, I have the following snippet of XML schema:

<xs:element name="Box" type="myXml:BoxType" minOccurs="0" maxOccurs="1"/>

<xs:complexType name="BoxType">
  <xs:sequence>
    <xs:element name="Ball" type="xs:string" minOccurs="0" maxOccurs="1"/>
    <xs:element name="Pen" type="xs:string" minOccurs="0" maxOccurs="unbounded"/>
  </xs:sequence>
</xs:complexType>

1st XML example:
  <Box>
    <Ball>Blue</Blue>
    <Pen>Red</Pen>
    <Pen>Black</Pen>
  </Box>

2nd XML example:
   <Box/>

   In the 2nd example, because <Box> element has no inner elements, I like <Box> element to not exist at all. How do I modify the above XML schema to do that? I like <Box> element to exist if it has at least 1 inner element, else no <Box> element at all.