RGSim Rely/ Guarantee Simulation (S,∑) (S,∑) (S,Σ") (S",∑”) G R ≤ ≤ T",o”") g , s(s,R,g)(T, ) (S, ) (S’, ’) (T’, ’) * (S’’ , ’’’) (T’’ , ’’’) e e * … … * R r G g G g RGSim = Rely/Guarantee + Simulation ≲ ≲ ≲ (S’ , ’’) (T’, ’’) ≲ (T, r, g) ≲ (S, R, G)