Daffodil doesn't support the XPath lower-case() function ... so what to use instead?

[Costello, Roger L.]

Hello DFDL community,

In the below snippet I use the XPath lower-case() function. I discovered, 
however, that Daffodil doesn't support lower-case(). Is there an alternative?  
/Roger

<xs:element name='signature' type='xs:string' dfdl:inputValueCalc='{
    if (lower-case(xs:string(../Hidden_signature)) eq "5a4d") then "Portable 
Executable (PE) file"
    else xs:string(../Hidden_signature)
    }' />