问映射hbm文件时出错“元素'class‘in具有无效子元素'set’in命名空间”

EN

 public class StudentDao
    {

        public virtual int Id { get; set; }
        public virtual string FirstName { get; set; }
        public virtual string LastName { get; set; }
        public virtual StudentDegreeType Degree { get; set; }
        public virtual string Field { get; set; }
        public virtual IEnumerable<StudentCourse> StudentCourses { get; set; }

    <?xml version="1.0" encoding="utf-8"?>
<hibernate-mapping assembly="Tosan.Sevrice.DataAccess" namespace="Tosan.Sevrice.DataAccess.Dao" xmlns="urn:nhibernate-mapping-2.2">
  <class name="StudentDao" table="STUDENTT" lazy="true" >
    <id name="Id" column="ID" >
      <generator class="increment"> </generator>
    </id>
    <discriminator column="Degree"/>

    <property name="FirstName" column="FIRSTNAME" />
    <property name="LastName" column="LASTNAME" />
    <!--<property name="Degree" column="DEGREE" />-->
    <property name="Field" column="FIELD" />

    <subclass name="MasterStudent" discriminator-value="1">
         <property name="َArticle"  />
    </subclass>
    <subclass name="BachelorStudent" discriminator-value="2">

    </subclass>

    <set name="StudentCourses" table="StudentCourse" inverse="true" cascade="all,delete-orphan">
      <key column="ID"/>
      <one-to-many class="StudentCourse"/>
    </set>

  </class>
</hibernate-mapping>

public class BachelorStudent : StudentDao
    {
    }
}

public class MasterStudent : StudentDao
    {
        public virtual bool Article { get; set; }
    }
}