Recently, Tsadiras and Margaritis have shown that the function implemented in the expert system MYCIN for combining certainty factors is a uniform. In this paper, we present a compact alternative to the given direct and lengthy proof, based on the construction of representable uninorms by means of an additive generator. Consequently, we show that van Melle's combining function is a representable uninorm. In fact, it is a rescaled version of the representable uninorm on the unit interval constructed from the algebraic product and the probabilistic sum.