Some distribution functions allow for a special input and output named store. This is a structure wherein the function can store the intermediate calculation results that it has done for a set of inputs, as its fields. These results might speed up things when calling another function of the distribution with the same set of inputs. For instance, the usual syntax for the ll function which calculates log-likelihoods is:
ll = D.ll(theta, data)
But this function also admits the following syntax:
[ll, store] = D.ll(theta, data, store)
where, either store is optional. If we also need the gradient of the log-likelihood (llgrad) for the same theta and data, we can use the store output from the ll function as an input to the llgrad function to prevent llgrad from re-calculating the intermediate values which are shared in the two functions:
[ll, store] = D.ll(theta, data); dll = D.llgrad(theta, data, store);
Please note that you should only share the store while working with the same distribution and inputs (theta and data). When these are changed, a new store structure should be used, or you will get wrong results.