Java SimpleXML 2.7 serialize/deserialize different objects are implemented one interface

How can I build from this model xml as below?

@Root
class Vertex {
    @Attribute(name = "id")
    private String _id;

    @Element(name = "x")
    private Expression _x;

    @Element(name = "y")
    private Expression _y;
} 

@Root
class Expression {
    @Element(name = "param", type = ParamReference.class, required = false)
    private ParamReferenceInterface _value;

    @Element(name = "sum", type = Sum.class, required = false)
    private SumInterface _sumValue;

    @Element(name = "fixed", type = FixedValue.class, required = false)
    private FixedValue _fixedValue;
}

<vertex id="someId">
   <x>
     <fixed>0.0</fixed>
   </x>
   <y>
     <sum>
       <param name="startPostion"/>
       <param name="height"/>
     <sum>
   </y>
</vertex>

Now I have XML like this:

<vertex id="someId">
   <x>
     <expression>
         <fixed>
             <fixed>0.0</fixed>
         </fixed>
     </expression>
   </x>
   <y>
      <sum>
         <expression>
            <param name="startPostion"/>
         </expression>
         <expression>
            <param name="height"/>
         </expression>
      </sum>
   </y>
</vertex>

It contains excess tags. I tried used Converter, I remove all annotations from Expression.class and I create ExpressionConverter, but I can't deal how write/read List values.

How can I hide 'expression' tags?