silverlight: set default impression generic.xaml child class?

i have following hierachy:

public category a
{
  stable category b
  {
  }
}

and i've attempted interpretation default impression following ways (inside generic.xaml):

<style targettype="local:a+b">
    <setter property="template">
        <setter.value>
            <controltemplate targettype="local:a+b">
                <grid/>
            </controltemplate>
        </setter.value>
    </setter>
</style>

<style targettype="local:a.b">
    <setter property="template">
        <setter.value>
            <controltemplate targettype="local:a.b">
                <grid/>
            </controltemplate>
        </setter.value>
    </setter>
</style>

<style targettype="local:b">
    <setter property="template">
        <setter.value>
            <controltemplate targettype="local:b">
                <grid/>
            </controltemplate>
        </setter.value>
    </setter>
</style>

does anyone know repremand syntax?