1. A natural language generation method for generating a referring noun phrase for an intended referent found in one or more messages within a document plan, the method comprising: arranging, using a processor, the one or more messages in the document plan, wherein messages represent a phrase or a simple sentence and are created in an instance in which the input data stream comprises data that satisfies one or more message requirements; identifying an intended referent in a message of the one or more messages to be referred to in a textual output; determining a lowest common ancestor for the intended referent and a previously referred-to entity within a part-of hierarchy; determining a salient ancestor of the intended referent within the part-of hierarchy; generating a referring noun phrase for the intended referent to be used in the textual output by traversing the part-of hierarchy from the salient ancestor to the lowest common ancestor, wherein the reference noun phrase comprises a default descriptor of the intended referent and one or more default descriptors of one or more parts of the part-of hierarchy that are traversed; and generating the textual output, wherein the textual output linguistically describes at least a portion of the input data stream.