when is inheritance from xs:anySimpleType allowed?

Hej,

in which case is it allowed to derive a simpleType or complexType from 
xs:anySimpleType?

I found the following cases:

    <xs:simpleType name="t1">
        <xs:restriction base="xs:anySimpleType" />
    </xs:simpleType>

    <xs:complexType name="ct1">
	<xs:simpleContent>
         <xs:extension base="xs:anySimpleType" />
        </xs:simpleContent>
    </xs:complexType>

    <xs:complexType name="ct2">
	<xs:simpleContent>
         <xs:restriction base="ct1" >
         </xs:restriction>
        </xs:simpleContent>
    </xs:complexType>

Are they all valid?

Ciao,
Tobias