rrd_calc and avoid repeated memory allocations/initial setups

Hi!

It seems as if there is no easy means to store extra data for the whole life-time of the
rpn_calc loop.

The reason why I ask is because I am implementing the predictperc function, which is 
similar to predict and predictsigma but it gives the X percentile.

For this I need to order the data (depending on arguments maybe 50-60 floats),
which means allocating memory (the exactly same amount for every time rpn_calc gets called)
to keep all those values sorted to find the correct percentile.

Unfortunately the way rrd_calc works, there is no data that is kept between calculations,
to avoid the repeated create/teardowns of this structures...

Seems as if an "easy" way would be enhancing rpnp_t by adding this this:
void *extra; /* extra data used during setup */
void (*free_extra)(void *); /* a function used to free the extra structure... 
			     * - if null, then a "normal free" is done */

and some code in rpnstack_free to call the "free" functionpointer with the argument of extra.
(or free directly if the pointer is NULL)

So would something like the above be acceptable?

Thanks,
	Martin