Compute a way to order a given set of dominoes in such a way that they form a
correct domino chain (the dots on one half of a stone match the dots on the
neighbouring half of an adjacent stone) and that dots on the halfs of the stones
which don't have a neighbour (the first and last stone) match eachother.

For example given the stones `21`, `23` and `13` you should compute something
like `12 23 31` or `32 21 13` or `13 32 21`, etc.

Compute a way to order a given set of dominoes in such a way that they form a
correct domino chain (the dots on one half of a stone match the dots on the
neighbouring half of an adjacent stone) and that dots on the halfs of the stones
which don't have a neighbour (the first and last stone) match each other.

For example given the stones `21`, `23` and `13` you should compute something
like `12 23 31` or `32 21 13` or `13 32 21`, etc.

Fix missing space between words
Fix missing space between words