 (‘Sizxy’ may be read: ‘z is a sumi of x and y’. The first notion is found e.g. in Eberle 1967, Bostock 1979, and van Benthem 1983; the second in Tarski 1935 and Lewis 1991; the third in Needham 1981, Simons 1987, and Casati and Varzi 1999.) Then, for each i ∈ {1, 2, 3}, one could extend M by adding a corresponding axiom as follows, where again ξ specifies a suitable binary condition: