... systems that are robust against incomplete KBs and inputs? Although robustness has been heavily stud- ied in natural language understanding (Weischedel and Black, 1980; Hayes, 1981; Lavie, 1994), ... singular, plural, and occasionally even proper nouns, while a does not. On the down side, the's only outnumber a's and an's by about two-to-one (Knight and Chander, 1994), so ... cal language modeling. We built a language model 4See also (Harbusch et al., 1994) for a thorough dis- cussion of defaulting in NLG systems. for the English language by estimating bigram and...