  I point out that the authors' interpretation of their calculations differs
from the standard interpretation, described in Sect. 84 of Landau-Lifshitz
book. This casts doubt on the authors' claim that Sagnac effect "arises also in
apparatuses which are fixed relative to the distant stars (i.e., to
asymptotically inertial frames); in this case one talks about frame-dragging".
