Corral (also known as "Bag", "Cave") is a logic puzzle invented by Nikoli. The aim is to draw a single closed loop along the grid lines that does not intersect itself. The loop goes around all numbers. The numbers in the grid indicate how many cells inside the loop can be seen horizontally and vertically from that cell, including the cell itself. Question marks can be used instead of clue numbers. Each question mark can represent any nonzero integer.