Unknown word sequences in HPSG

This work consists of an investigation into the properties of unknown words in HPSG, and in particular into the phenomenon of multi-word unknown expressions consisting of multiple unknown words in a sequence. The work presented consists first of a study determining the relative frequency of multi-word unknown expressions, and then a survey of the efficacy of a variety of techniques for handling these expressions. The techniques presented consist of modified versions of techniques from the existing unknown-word prediction literature as well as novel techniques, and they are evaluated with a specific concern for how they fare in the context of sentences with many unknown words and long unknown sequences.