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.
13. A method according to claim 12 , wherein: determining a first hash value includes retrieving data representing the graph from machine-readable media and also retrieving a verification hash value from machine-readable media, verifying authenticity of the data representing the graph by locally computing a hash value based upon retrieved data representing the graph, and determining whether the verification hash value matches the locally-computed first hash value, and if the authenticity of the data is verified, using the matched results as the first hash value and otherwise generating an error; and adding a new statement to the data by storing the new statement on the machine-readable media, by computing as the second hash value an incremental hash value representing the new statement, and by storing the aggregate hash value on the machine-readable media in association with the graph, where the aggregate hash value represents addition of the first hash value and the second hash value.