Equating variables

I was scanning the MathML documentation looking for a way to say

x = 5y -13.

using Content Markup.  Relation tags do not work since they are supposed to be rendered as true of false.  Using the Declare tags work in a fashion but they actually render as a definition of x instead of an operation of assignment of the result of 5y-13.

I have seen another tag that was not mentioned in the MathML documentation. (See http://irt.org/articles/js081/index.htm )  It is called <e> for 'equation or relation' but that would break the initial rule of having relational operations render as true or false.

Any suggestions?  Is there new documentation than from April 7, 1998?

I was considering using <eq/> as an operator

<apply>
    <eq/>
    <ci> x </ci>
    <apply>
        <minus/>
            <apply>
                <times/>
                    <cn> 5 </cn>
                    <ci> y </ci>
            </apply>
            <cn> -13 </cn>
    </apply>
</apply>

If right, this would be a simple solution for my mathematical content evaluator.

Thanks,

Dr. Walter T. Stephens