Mark Levi gratefully acknowledges support by the NSF grant DMS-0605878. Oleg Kirillov
has benefited from the ERC Advanced Grant ‘Instabilities and nonlocal multiscale modelling
of materials’ FP7-PEOPLE-IDEAS-ERC-2013-AdG (2014–2019). The authors are grateful
to the anonymous referee for the useful suggestions which helped improve the exposition.