Extending addition in Elliott's local semigroup

We study the unique extendability of Elliott′s partial addition of Murray-von Neumann equivalence classes of projections in AF C*-algebras. We prove that there is at most one commutative associative monotone extension satisfying the natural residuation condition that for each projection p the class of 1 - p is the smallest one whose sum with the class of p equals 1. We prove that for every AF C*-algebra A this associative commutative monotone residual extension exists if, and only if, the Murray-von Neumann order on equivalence classes of projections in A is a lattice order. By Elliott′s classification theorem, the resulting monoid uniquely characterizes A. We give a simple equational characterization of the monoids arising as classifiers.