Two consecutive derivations by restriction

Hallo,
 
I hope, that this has not been answered before, but I couldn´t find a hint
in this list.
My questions are:
 
First question:
 
Is it possible, to perform two or more consecutive derivations by
restriction?
 
Given the following schema:
 
<?xml version="1.0"?>
<xsd:schema xmlns:xsd=" http://www.w3.org/2001/XMLSchema
<http://www.w3.org/2001/XMLSchema> ">
   
   <xsd:element name="Liste">
      <xsd:complexType>
         <xsd:sequence>
            <xsd:element name="Firma" type="FirmaType" />
            <xsd:element name="Operator" type="OperatorType" />     
         </xsd:sequence>
      </xsd:complexType>
   </xsd:element>
   
   <xsd:complexType name="ListType">
      <xsd:sequence>
         <xsd:element name="Name" type="xsd:string" />
         <xsd:element name="Comment" type="xsd:string" minOccurs="0"
maxOccurs="3"/>
         <xsd:element name="Street" type="xsd:string" />
      </xsd:sequence>
   </xsd:complexType>
   
   <xsd:complexType name="FirmaType">
      <xsd:complexContent>
        <xsd:restriction base="ListType">
           <xsd:sequence>
              <xsd:element name="Name" type="xsd:string" />
              <xsd:element name="Comment" type="xsd:string" minOccurs="0"
maxOccurs="2" /> 
              <xsd:element name="Street" type="xsd:string" />
           </xsd:sequence>   
        </xsd:restriction>
      </xsd:complexContent>
   </xsd:complexType>
  
   <xsd:complexType name="OperatorType">
      <xsd:complexContent>
         <xsd:restriction base="FirmaType">
            <xsd:sequence>
               <xsd:element name="Name" type="xsd:string" />
            </xsd:sequence>
         </xsd:restriction>
      </xsd:complexContent>
   </xsd:complexType>    
   
</xsd:schema>
 
Although I am not 100% sure I don´t think it is, because this schema
produces an
"invalid particle derivation by restriction". I can not find any other
mistake in it.
 
If my assumption is correct I would also be interested to know, why this is
forbidden?
 
Second question: I tried to find some kind of confirmation in the
"Structures" for this problem.
 
If one was able to give me a pointer in Structures, I would be very
thankful.
 
Regards
 
Rainer Becker