Maybe they're unwritten, or lost in the depths of antiquity, but here they are, IIRC:

Sparteye's Game, as originally conceived: Use two particular given words in a short poem. Then present your own pair of words for the next person to use similarly.

-- The verse is [usually] in the form of a limerick, with its standard rhythm (the most frequently-bent rule)

-- The words to be used are not arbitrary; they come from some word-centered dictionary, or thesaurus, or such. They should be "index words," the first and last words on a single page.

The challenge in the game is to find word pairs that will be difficult (read "interesting," if you like) to incorporate into a single coherent short verse, not to bring the game to a screeching halt. Impossible words really aren't that hard to come by; there's no virtue in it.