How to handle a where-list in MathML

I've been involved in the development of a mathematical modelling tool 
which permits statements of the form:
   expression WHERE assignment1, assignment2
in order to simplify complex expressions.

Example:
   2*a+3*a^2+b+b^2 WHERE a=5+c*2, b=17-d

I'd like to represent the whole statement as a single MathML structure.

My intention is to use
<apply>
   <ci>WHERE</ci>
   expression
   assignment1
   assignment2
   ....
   assignmentn
</apply>
where 'expression', 'assignment1', 'assignment2' etc stand for the 
appropriate content MathML.

Is this an acceptable solution, or is there a better way of doing this?

Thanks!
Robert Muetzelfeldt