1. A system comprising: a user interface device; and one or more server computers operable to interact with the user interface device and to: apply a pre-defined rule set to a plurality of versions of a first document in a plurality of structured documents to extract one or more index values, the pre-defined rule set including a plurality of rules, each rule having a distinct rule identifier, each extracted index value being extracted by a rule in the pre-defined rule set, wherein one or more versions of the first document is concurrently accessible to a plurality of users for collaborative authoring; and for each extracted index value, store in an index-value data structure the extracted index-value, the rule identifier of the rule that extracted the index value, and information identifying the first document and the respective version of the first document from which the index-value was extracted.