  It is argued that the tetrad in a recent paper by Porto and Rothstein on
gravitational spin-spin coupling should not have the given form. The fixation
of that tetrad was suggested by Steinhoff, Hergt, and Schaefer as a possible
source for the disagreement found in the spin-squared dynamics. However, this
inconsistency will only show up in the next-to-leading order spin-orbit
dynamics and not in the spin-squared dynamics. Instead, the disagreement found
at the next-to-leading order spin-squared level is due to a sign typo in the
spin-squared paper by Porto and Rothstein.
