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Explanation of DRS threading.
Now that we are familiar with the syntax and semantics of the DRS language and with coding DRSs in Prolog, it is time to implement DRT for a fragment of English. Working with DRSs, as we will see shortly, is rather different than working with first-order formulas, and we will need to introduce some new machinery for constructing DRSs and performing pronoun resolution. Nevertheless, we will be able to reuse a lot of the software engineering work we did in the previous chapters - we will still be using semantic macros and semantic combination rules, and we will essentially use the same lexicon and grammar as before.
However, the way we will construct DRSs will be radically different from what we have seen before. We will use a method call threading, due to Johnson and Klein, a very intuitive approach to building semantic representations. Given a syntactic tree of an English expression, a DRS is threaded around the nodes of this tree in a left-to-right top-down way, thereby accumulating information in the DRS as it goes along.
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