How to derive from a mixed anyType?

Given a base type defined thus:

  <complexType name="AbstractBaseType" abstract="true" mixed="true">
    <attribute name="id" type="ID" use="optional"/>
  </complexType>

The intention was to define a type from which both simpleContent and
complexContent types could be derived, which may carry an id attribute.  

1.  how does one derive an element-only type from it?  I think extension
leaves the mixed content still there.  does it require a two-step
restriction to suppress the literal content then extension to add the
desired element content?  What does this look like?

2.  how does one derive a simpleContent type whose content has a specified
simpleType?