Draw a loop passing through centres of cells horizontally or vertically, passing through all circles. The loop must cross itself, but only in cells with a given '+'. The loop must pass straight through 'stations', i.e. numbered cells, and must travel in order from 1 to 2 and so on till N, where N is the highest number that will be placed in the grid, and then back to 1. No numbers and crosses can be placed in the grid apart from those indicated within the yarns.
(Rules and example from WPC 2017 IB)
History of the puzzle
First appeared in WPC 2000/Part I. Invented by Craig Kasper (Canada).