mo attributes?

MathML discusses at http://www.w3.org/TR/REC-MathML/chap3_2.html the
attributes of the mo elements.  In particular, for the `form' attribute 
the following values are offered.

  	prefix | infix | postfix

Is it possible to provide a `form' value to a mo element that appears
in isolation? For instance, in the case

      <mtd> <mo form="...">=</mo> </mtd>

motivated by the following example.

     \begin{eqnarray*}  
     a &=& b \\  
     c &=& d 
     \end{eqnarray*}
     
     
     <mtable columnalign="right center left">
       <mtr>
         <mtd> <mi>a</mi> </mtd>
         <mtd> <mo>=</mo> </mtd>
         <mtd> <mi>b</mi> </mtd>
       </mtr>
       <mtr>
         <mtd> <mi>c</mi> </mtd>
         <mtd> <mo>=</mo> </mtd>
         <mtd> <mi>d</mi> </mtd>
       </mtr>
     </mtable>
     
Also, is there any problem with the following code?

     <mo form="prefix" fence="true" stretchy="true" lspace="0em" rspace="0em"> ( </mo> 
     <mi>x</mi> 
     <mo form="postfix" fence="true" stretchy="true" lspace="0em" rspace="0em"> ) </mo>
     
OpenOffice rejects the above MathML code fragments.

-eitan