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The objective, as with several similar puzzles, is to draw a single loop through all available cells in the grid, from the given clues.
A typical puzzle consists of a 10x10 grid. In the grid are clue cells, each containing a number and an arrow. A clue cell tells the solver how many shaded cells there are in a particular direction, from that point. If a cell is not a clue cell and does not contain a shaded cell, then it will contain a segment of the loop.
An intriguing aspect of this puzzle is the requirement to place shaded cells, in addition to the loop. This process feels almost haphazard, at first, but the puzzle has a unique solution.
Yajilin is a Japanese puzzle that made its first appearance in a Nikoli title (Puzzle Communication Nikoli, Vol 86, June 1999).
The example puzzle, with a neat layout, is supplied by Johannes Kestler (PuzLogical).