Beginning with the initial state of a single carbon atom (i.e., methane after adding implicit hydrogens), possible actions were enumerated following a series of expansion and filtering steps. First, all possible tautomers of the given starting molecule were considered as possible starting states. From each starting state, a new bond was added between an atom in the molecule and a second atom, either already in the molecule (forming a ring) for an unbonded C, N, O, or S atom. New molecules were generated for every possible atom pair and bond type (single, double, or triple) for which valency rules were satisfied. From this set of all possible next actions, molecules were filtered according to several ring, saturation, and synthetic accessibility criteria, including restricting molecules to a maximum SAScore of 4.0. The action space was then further expanded by enumerating all possible stereochemical configurations of the starting molecule, followed by a reduction to canonical tautomer forms. Next actions were then de-duplicated by SMILES string.