XML : XML Schema - declare with type=anything

Is there any way in XSD schema to define an element, which can be anything. xs:anySimpleType will not allow complex (nested) elements. xs:anyType will try to match with any declared types in xsd.

Is there any way to allow anything in xml that can have anything, like -

  <xs:element name="a" type="xs:anything"/>    

so, element 'a' can be anything like -

  <a />    

or

  <a some_attribute="" />    

or

  <a>      <b />      <c />  </a>