UNLESS you're Minnesota Fats, it pays to give Ho Chi Minh City's pool halls and billiards parlours a wide berth if you're seeking a relaxing evening. Both games are becoming extremely popular across the city and all manner of darkened bars, clubs and halls are springing up to attract the young macho types who nightly stake their identities on a frame or two. Given the custom, they can be tense arenas indeed. First there are the rules to be negotiated. Like Vietnamese chess - a hybrid which involves a river that winds across the board to make things even more complicated - nothing is simple or even consistent. It pays to sort it all out beforehand to ensure a clear head to deal with the next problem - table etiquette, or lack thereof. It is here that the hoods ensure their version of pool has a theatre all its own beneath the bald light bulb and slow twirling ceiling fans. There is noise, heat and often little room to move. And, as in classic guerilla warfare, any advantage is seized upon to torment and terrorise an enemy. Little heed is paid to table conventions considered standard elsewhere. An opponent is free to slam his chalk down on the table edge with a resounding thwack just as a player is poised to shoot. He might blow smoke from his dangling cigarette across the table, or worse, let it fall from his lips on to the table, creating a momentary fire scare. His friends may gather slightly menacingly over a particularly strategic pocket. They are not above offering unsolicited advice or even extending a painted pinkie finger next to the target ball by way of a guide. A particularly curvaceous girlfriend or bar girl may be summoned to rest on her elbows and lean out into the light over the table. Then, finally, when the concentration is restored and the eight-ball about to be potted, don't be surprised if a fuse blows and the place is plunged into darkness, save from the glowing fag-ends of the heavies surrounding the table. Generally, most of the punters seem to be culled from the growing ranks of young male 'under-employeds' but who always turn out in sharp suits, shiny shoes and are prone to wearing chunky university graduation rings. Vietnamese mothers generally warn their young daughters to stay away from such establishments. Given the atmosphere, police are never far away. In a scene straight from Casablanca, this correspondent has seen a pot-bellied police chief march into one establishment, complain that gambling was taking place before ordering his goons to seize all the balls from the table. It was the local district officers' championships that weekend.