It was recalled that the topological classification of defects in quasicrystals was incomplete. If the orthogonal space E was regarded as order parameter space, then it was not a connected one. This was because faces, edges and vertices of the projections of the atomic surfaces had to be omitted in order to avoid ambiguities in the quasi-crystalline structure. This reduction of E gave rise to wall, line and point singularities. In particular, so-called phason singularities were parts of walls. It was pointed out that global and local changes in the phason variable could permit vertices to be transported along loops or along paths which extended to infinity.

A.Trub, H.R.Trebin: Journal de Physique I, 1994, 4[11], 1855-66