Working on implementing ADM and BSSN 3+1 formalism. I coded this up after reading through a few books and papers on Numerical Relativity. These books and papers all tend to jump right into the math required to write the variables in terms of their partial time derivatives, but don't seem to get to the hyperbolicity requirements for numerically modeling the equations until later. Key note.
Browse the FTCS BSSN source code here.