Meaning of fixed attribute on pattern facet where more than one pattern element is present and not all say fixed

Hi,

     In the XML representation of pattern facet [section 5.2.4 of Datatypes
Spec ],  "fixed" attribute is allowed on <pattern> element. However section
4.2.4  does not define a "fixed" property for  pattern facet. The same is
true for enumeration facet.

     So my question is :  is fixed attribute allowed in a pattern facet? If
the answer is yes, I would like to know the meaning of  fixed attribute on
pattern facet when more than one "pattern" element is used and  at least
one  says "fixed='true' " and another  says "fixed='false'".

     example :
     <simpleType name="char_between_a_and_d">
          <restriction base="string">
               <pattern value="a" fixed="true"/>
               <pattern value="b" fixed="true"/>
               <pattern value="c" fixed="false"/>
               <pattern value="d" fixed="false"/>
          </restriction>
     </simpleType>

     Does it mean that subtypes of char_between_a_and_d must always specify
<pattern value="a" /> and <pattern value="b" >?


Thank you.

Achille Fokoue