 We then define formula_217 as the opposite of this point.Hence in the case formula_218 we have that these three points are collinear, thus there is some linear formula_219 such that formula_32, formula_214 and formula_222 satisfy formula_223.Now, formula_224 which is the identity element of formula_199 as formula_226 is the divisor on the rational function formula_16 and thus it is a principal divisor.