Comment (by Zach Etienne):

Trying to adjust the Gamma-driving shift eta parameter is not straightforward. (I’m using the 2nd-order version.) I’ve only figured it out in the past by reading the ML_BSSN source code. Documenting this would be very much appreciated.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/880/improve-mclachlan-documentation