cs String Literal

I am considering using MathML v3 to represent conditional logic within
another XML vocabulary. I can see that there is a <condition> element
and a set of useful operators which can be used with an <apply>.
However, not all my conditions relate to numeric values, some are
dates and others just string comparisons. I noted that there is a <cs>
element which looks like it can be used to contain string literals,
but when I attempt to use it, it will not validate against the mathml3
XSD.

I want to do something like this, can anyone suggest a way this can be
acheived :-

<condition>
    <apply>
        <eq/>
        <ci>PolicyNumber</ci>
        <cs>abc123</cs>
    </apply>
</condition>

or ...

<condition>
    <apply>
        <geq/>
        <ci>PolicyEffectiveDate</ci>
        <cs>2011-01-01</cs>
    </apply>
</condition>

I am happy to write my own parser for this, but I want to create a
valid fragment according to the MathML schema rather than something
which is not.

Thanks

Fraser.