0 QTRS
+(a, b) → +(b, a) +(a, +(b, z)) → +(b, +(a, z)) +(+(x, y), z) → +(x, +(y, z)) f(a, y) → a f(b, y) → b f(+(x, y), z) → +(f(x, z), f(y, z))