Question

Let D be the digraph whose vertices are the pairs of integers 11, 12, 13,21,22,23,31,32,33,
and whose arcs join ij to Id if and only if j = k. Find an Eulerian trail in D and use it to obtain a circular arrangement of nine Is, nine 2s and nine 3s in which each of the 27 possible triples (111, 233, etc.) occurs exactly once.