Soak tamarind in 1/2 litre of water in a thick bottomed vessel and keep aside. Heat a little oil in a frying pan and fry urad dal, pepper,red chillies,asafoetida powder. Take out this fried mixture adda little water and grind into smooth paste and keep aside. Fry bitter-nut vetral/manthakkaali vetral ina little oil and keep aside. Pour a little water on the rice flour so that it just dissolves mix well and keep aside.Squeeze out the tamarind, retaining the tamarind and water solution. Add curry leaves and bring this solution to boil.Add fried vetral to this and allow to simmer until the aroma of tamarind subsides.Add rice flour and water solution to this. Remove from fire when the solution thickens.Heat a little oil and fry mustard, curry leaves and season the tamarind solution with this mixture.Add salt to taste. Serve hot with rice.