"The Murphy Rule", by Paul Korngiebel
League rule 18.4 subsection A line 4: ". . . he who headbutteth the enemy in an unseemly fashion and scampereth away, thus discrediting the valor of his team, shall be suspended forewith and with posthaste dispatch forfeit said right to play against all other enemies for a period of five league matches, including any fun games that might lessen his suffering from being banned, but no longer than the menstrual cycle of a yak, lest such dastardly and cowardly behavior infect his team."
Thus spaketh the Lord of SASL, and the players, though filled with blood lust for the enemy, knew it was good and right, and so gave up their desire to headbutteth the enemy, even when they knew the enemy deserved a good knock on the noggineth.
And thus football became distinct from rugby.