Writing own aggregation functions in R

Writing own aggregation functions in R: That would be a tremendous way to extend Spotfire. Expression functions are nice, but they cannot be used with OVER functions. So if an expression function should be used on sliced data, the slicing has to be implemented in the expression function somehow which I find clumsy. Imagine for example a set of measurement curves for different experiments. If I would like to calculate the numerical integral per experiment, I have to slice in R somehow. If I could define my own aggregation function, I could just write something like the following in a calculated column expression: MyOwnNumericalIntegration([xvalues],[yvalues]) over [Experiment]. That would also allow for easy curve fitting, interpolation (everything easily available in R) etc. on sliced data