Grammar Engineering Frequently Asked Questions
When I look at the parse chart, I don't see an edge that I'm expecting to be there. How do I find out why it's missing? When you don't see an edge that you expect to be there (for example, an edge licensed by the head-complement phrase spanning a verb and its complement NP), this is a job for interactive unification. Since the parser works bottom up, an edge can't be in the chart if its daughters aren't there. For example, if you're expecting a subtree like this:
VP / \ V NP | N
but the VP is missing, the first thing to check is whether the V and NP nodes are there. That is, look for the lowest edge that you expect to find but don't. Now, find the edge(s) that you expect to be the daughter(s) of the missing edge and determine which rule (grammar rule or lexical rule) should be licensing the edge. Then, follow the directions for doing interactive unification.