[1] Univ Oxford, Dept Comp Sci, Oxford OX1 2JD, England
[2] Queensland Univ Technol, Math Sci, Brisbane, Qld 4072, Australia
[3] Univ Queensland, Sch Math & Phys, Brisbane, Qld 4072, Australia
[4] Queensland Univ Technol, ARC Ctr Excellence Math & Stat Frontiers, ACEMS, Brisbane, Qld 4072, Australia