validatable.xsd

<?xml version="1.0" encoding="UTF-8"?>

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="wrapper" type="sequence"/>
  <xs:complexType name="sequence">
    <xs:sequence>
      <xs:element name="first" type="xs:string"/>
      <xs:element name="second_opt" minOccurs="0" type="xs:string"/>
      <xs:element name="third" type="xs:int" />
      <xs:element name="fourth_0_2" type="xs:string" maxOccurs="2" minOccurs="0"/>
    </xs:sequence>
    <xs:attribute name="attr_opt" type="xs:string"/>
    <xs:attribute name="attr_req" type="xs:string" use="required"/>
  </xs:complexType>
</xs:schema>