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Significant update to my reference page on "Conventions, Definitions, Identities, and Formulas.”

Changes to:
* Hypersurfaces with a spacelike or timelike unit normal
* Small variations of the metric

This is a collection of useful results for folks working in gravitational physics and high-energy theory. It's a repository for things I frequently need, and includes some explicit results which (as far as I am aware) do not appear anywhere in the literature.

http://jacobi.luc.edu/Useful.html