FW: howto: get the value from a multivalued field?

[world hello]

hi, all - 
how can I retrieve the value out of a multivalued field in a customized 
function query?I want to implement a function query whose first parameter is a 
multi-value fileld, from which values are retrieved and manipulated. 
however, I used the code but get exceptions - "can not use FieldCache on 
multivalued field"
/////////////////////////////public ValueSource parse(FunctionQParser fp) 
throws ParseException {
        try {             ValueSource vs = fp.parseValueSource();       }       
catch (...)       {       }////////////////////////////
Thanks.
- Frank