XML : XSD SaxParse exception

I have xsd file:

  <?xml version="1.0" encoding="UTF-8" ?>  <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">   <xs:element name="triptype">    <xs:complexType>      <xs:sequence>     <!--definition of simple elements -->     <xs:element name="country" type="xs:string"/>     <xs:element name="days" type="xs:integer"/>          <!--definition of attributes -->        <xs:extension base="xs:string">               <xs:attribute name="tripid" use="required" type="xs:ID"/>        </xs:extension>     <!-- definition of complex elements -->          <xs:simpleType name="type" final="restriction">      <xs:restriction base="xs:string">        <xs:enumeration value="day_off" />        <xs:enumeration value="excursion" />        <xs:enumeration value="rest" />        <xs:enumeration value="pilgrimage" />      </xs:restriction>     </xs:simpleType>     <xs:simpleType name="transport" final="restriction">      <xs:restriction base="xs:string">        <xs:enumeration value="avia" />        <xs:enumeration value="railway" />        <xs:enumeration value="auto" />        <xs:enumeration value="liner" />      </xs:restriction>     </xs:simpleType>       <xs:element name="cost">      <xs:complexType>       <xs:sequence>         <xs:element name="service" type="xs:decimal" />         <xs:element name="road" type="xs:decimal" />         <xs:element name="food" type="xs:decimal" />         <xs:element name="excursion" type="xs:decimal" />         </xs:sequence>      </xs:complexType>     </xs:element>      </xs:sequence>   </xs:complexType>   </xs:element>  </xs:schema>  

When i validate it i get errors: Not valid. Error - Line 11, 35: org.xml.sax.SAXParseException; lineNumber: 11; columnNumber: 35; s4s-elt-must-match.1: The content of 'sequence' must match (annotation?, (element | group | choice | sequence | any)). A problem was found starting at: extension. Error - Line 17, 51: org.xml.sax.SAXParseException; lineNumber: 17; columnNumber: 51; s4s-elt-must-match.1: The content of 'sequence' must match (annotation?, (element | group | choice | sequence | any)). A problem was found starting at: simpleType. Error - Line 25, 56: org.xml.sax.SAXParseException; lineNumber: 25; columnNumber: 56; s4s-elt-must-match.1: The content of 'sequence' must match (annotation?, (element | group | choice | sequence | any)). A problem was found starting at: simpleType.*

What is the problem?