No faster-than-light observers (GenRel)

We have previously verified, in the first order theory SpecRel of Special Relativity, that inertial observers cannot travel faster than light. We now prove the corresponding result for GenRel, the first-order theory of General Relativity. Specifically, we prove that whenever an observer m encounters another observer k (so that m and k are both present at some spacetime location x), k will necessarily be observed by m to be traveling at less than light speed.