12. A method of processing data representing a graph, the graph being stored on machine-readable media, said method comprising: determining a first hash value representing the graph; using a digital processor to compute a second hash value that represents an additional statement; and computing an aggregate hash value that is a function of each of the first hash value and the second hash value, where the function is commutative.
12. A method of processing data representing a graph, the graph being stored on machine-readable media, said method comprising: determining a first hash value representing the graph; using a digital processor to compute a second hash value that represents an additional statement; and computing an aggregate hash value that is a function of each of the first hash value and the second hash value, where the function is commutative. 15. A method according to claim 12 , further comprising: determining an attribute of all data representing the graph, including the additional statement; generating an aggregate value dependent upon both the aggregate hash and the attribute; and digitally signing the aggregate value to thereby generate a digital signature.