  The tetrad-based equations for vacuum gravity published by Estabrook,
Robinson, and Wahlquist are simplified and adapted for numerical relativity. We
show that the evolution equations as partial differential equations for the
Ricci rotation coefficients constitute a rather simple first-order
symmetrizable hyperbolic system, not only for the Nester gauge condition on the
acceleration and angular velocity of the tetrad frames considered by Estabrook
et al., but also for the Lorentz gauge condition of van Putten and Eardley, and
for a fixed gauge condition. We introduce a lapse function and a shift vector
to allow general coordinate evolution relative to the timelike congruence
defined by the tetrad vector field.
