This, in turn, specifies a (bipartite) matching between vertices of X i and Y that correspond to the same vertex of M i . In chemical terms, this matching is a partial atom-atom map between X i any Y and thus also between X and Y . To characterize the part of X i that does not match Y in more detail, we consider the subgraph reactions. In principle, it can be applied to product-dominant reactions as well. However, imputing a missing reactant is more challenging than finding a missing product.