distribution

<xs:attribute name="distribution">
  <xs:simpleType>
    <xs:restriction base="xs:string">
      <xs:enumeration value="binomial"/>
      <xs:enumeration value="gamma"/>
      <xs:enumeration value="igauss"/>
      <xs:enumeration value="negbin"/>
      <xs:enumeration value="normal"/>
      <xs:enumeration value="poisson"/>
      <xs:enumeration value="tweedie"/>
    </xs:restriction>
  </xs:simpleType>
</xs:attribute>