“…On the other hand, (21) informs us that the total normal pressure must vanish on the matching surface, which again is a very satisfactory result and generalizes similar properties in the static case. In fact, (21) is strictly equivalent to the matching condition (10). In order to prove this, let us note that (21) is generated by which via Einstein's equations can be rewritten in terms of the Einstein tensor GI,, as Now, by using (A81, ( 1 11, and (AI)-(A4), it is straightforward to show that this last relation, and therefore (21), leads exactly to the matching condition (10).…”