User Defined Aggregations in MonetDB?

Dear all, I was wondering if it is possible to build a User Defined Aggregation in MonetDB. For example, rather than do SUM(myfunction(a,b)) I would like to do MYAGG(a,b) where MYAGG internally holds a small buffer of a and b values so that they can be calculated in a batch fashion with a high performance batch implementation of the function. Could MYAGG be implemented in C? Best regards, Alastair