18. A non-transitory computer readable memory medium having program code instructions stored thereon, the program code instructions which when executed by an apparatus cause the apparatus at least to: arrange one or more messages in a document plan, wherein messages represent a phrase or a simple sentence and are created in an instance in which an input data stream comprises data that satisfies one or more message requirements, wherein at least a portion of the input data stream comprises non-linguistic data; identify an intended referent in a message of the one or more messages to be referred to in a textual output; determine a lowest common ancestor for the intended referent and a preyiously referred-to entity within a part-of hierarchy; determine a salient ancestor of the intended referent within the part-of hierarchy; generate a referring noun phrase for the intended referent to be included in a textual output by traversing the part-of hierarchy from the salient ancestor to the lowest common ancestor such that a default descriptor is added to a queue for at least a portion of entities traversed in the part-of-hierarchy, 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; generate the textual output comprising the referring noun phrase such that it is displayable on a user interface, wherein the textual output linguistically describes at least a portion of the input data stream; and display the textual output via a display device.
21. A computer program product according to claim 18 , further comprising program code instructions configured to: determine the previously referred-to entity based on a last mentioned entity in a discourse model.