  Given an d-dimensional manifold with two commuting Killing vectors, together
with an d - 1 dimensional submanifold in which one of the Killing vectors lies,
then the lapse and shift of the second Killing vector, relative to this slice,
remain constant along the orbits of the `surface' Killing vector.
Alternatively, the six dot products that can be formed from the three vectors,
the two Killing vectors and the normal to the submanifold, are all constants
along the `surface' Killing vector.
