1. A method of managing multiple resource identifiers in a machine readable document, comprising: allocating one or more resource identifiers to a context, each resource identifier identifying where a corresponding resource is available for retrieval, the context specifying a common name for a group of resources, wherein the context comprises a plurality of sub-set contexts, each of the plurality of sub-set contexts being allocated to different resource identifiers and resolving different resources available for retrieval; assigning a context name to the context; associating the one or more resource identifiers and corresponding context name in the document; applying a transformation to the machine readable document, wherein the transformation affects the context; and updating the corresponding resource identifier associated with the context without updating the plurality of sub-set contexts.