by  Colin Vout

Published Sunday November 08 2026 (link)

Changing underground lines, we used a step-free access route: three lifts joining ends of pairs from four horizontal L-shaped corridors at different levels; the corridor segments ran on E-W or N-S axes, with a right-hand turn between the first and second.

We started on the green corridor, walking 20 yards, turning right, and going 40 yards. This was at a lower level than the yellow one (30 yards, turning right then 50 yards); for blue, the values were 60 then 10; for red, 70 then 20. I’ve forgotten the direction the corridors started in, and the order of the last three. The route ended exactly above where we started.

The arrangement had me in a knot — literally, because the route we followed actually formed a knot.

What was the order of the corridors (eg, BGRY), and the order of their levels (bottom to top)?