We also enhance a basic chart parser to handle features. This area overlaps syntax and semantics. A feature is a piece of information associated with a word. Feature systems serve a number of goals.
Features have a name and a value.
In this case the name is AGR and the value might be s or p. Some verbs have several allowable complements. See Figures 4.2 and 4.4 in Allen for more examples of complement structures. A head feature is one for which the feature value on a parent category must be the same as the value on the head subconstituent. This grammar could be expanded to show all the feature values.