Filtered expansions in general relativity I

This is the first of two papers in which we construct formal power series solutions in external parameters to the vacuum Einstein equations, implementing one bounce for the Belinskii-Khalatnikov-Lifshitz (BKL) proposal for spatially inhomogeneous spacetimes. We use a graded Lie algebra, homological framework. A dedicated filtration encodes key features of the BKL proposal, and we use it to set up an unobstructed perturbative problem.