pub fn positions(kinds: &[Kind]) -> Vec<(usize, usize)>Expand description
Where each image sits among the others of its own kind.
Answers, for every image in the order given, (index, of) — its place
among the images of that same kind, and how many there are. That pair is
what image_name needs, and computing it here rather than in each caller
is what keeps a downloaded booklet and an extracted one named alike.