Nix should automatically recover the Berkeley DB database. Nix should automatically remove Berkeley DB logfiles. Unify the concepts of successors and substitutes into a general notion of equivalent expressions. Expressions are equivalent if they have the same target paths with the same identifiers. However, even though they are functionally equivalent, they may differ stronly with respect to their performance characteristics. For example, realising a slice is more efficient that realising the derivation from which that slice was produced. On the other hand, distributing sources may be more efficient (storage- or bandwidth-wise) than distributing binaries. So we need to be able to attach weigths or priorities or performance annotations to expressions; Nix can then choose the most efficient expression dependent on the context.