For complex, real world sentences of the sort that are found in newspapers, no parser in existence can find full parses for more than 75% or so of the sentences. Therefore, these systems need ways of combining the parse tree fragments that they obtain. This module may be applied to the parse tree fragments themselves. Alternatively, each fragment is translated into a logical form fragment, and this module tries to combine the logical form fragments. One method of combination is simply to take the logical form of the whole sentence to be the conjunction of the logical form fragments. A more informative technique is to attempt to fit some of the fragments into unfilled roles in other fragments.
The methods that have been employed so far for this operation are ad hoc. There is no real theory of it. The methods are developed manually.