Peel the outer skin from the green papaya and finely shred the flesh on a cheese grater or chop very finely into long thin shreds. Set aside. In a mortar, lightly puond the garlic, add the chilis and lightly pound while occasionally stirring with a spoon to prevent the resulting paste from thickening. Add the long beans and slightly bruise them. Add the shredded papaya, lightly pound and stir until all the ingredients are blended together. Add the lemon juice, soy sauce and sugar and stir into the mixture. Finally add the tomato, stirring once. Arrange the Chinese cabbage leaves on a serving dish and turn the yam on to them. Diners should tear off a section of cabbage leaf to use as a scoop for the yam, the two being eaten together. This dish is especially good with sticky rice.