Draw a single loop consisting of vertical and horizontal line segments, so that it passes through the centre of each cell exactly once. The loop can never cross itself. The loop must enter and exit each contiguous outlined region exactly twice.
(Rules from WPC 2016 IB, example by EctoPlasma)
History of the puzzle
First appeared on Palmer Mebane's (USA) blog in 2009. 
See EntryExit for an independent, very similar puzzle.
Appearances in the past WPCs
- WPC 2016/Round 2 by Matej Uher