Thanks for all of these

I assume you wouldn't mind me merging the solutions into my 'master' POD, with credit of course.

Just from reading your solutions I learned a number of new functions, including maximum_ind (and it's sister functions maximum_n_ind, minimum_ind, minimum_n_ind and max2d_ind) and lags. This is what I hoped would happen, as everyone seems to know a different subset of PDL functionality.