  Let S be a bounded, Riemann measurable set in R^d, and L be a lattice. By a
theorem of Fuglede, if S tiles R^d with translation set L, then S has an
orthogonal basis of exponentials. We show that, under the more general
condition that S multi-tiles R^d with translation set L, S has a Riesz basis of
exponentials. The proof is based on Meyer's quasicrystals.
