Semantic Parsing

A sentence that parses to a tree structure with all the arguments of each predicate filled can be said to be semantically complete. However, with a tree structure of nested predicates, it's possible that some sentences, even grammatically correct ones, will parse to structures having blank arguments. This can be explained by the fact that contextual information may have been already delivered earlier in a document and that the author can then exercise brevity. This brevity may be more commonplace in informal spoken language.

Example:
Tommy put the book on the shelf in order to help.
InOrderTo(Did(tommy,PutOn(book,shelf)),Help(tommy,_))

The context of a previous sentence in a document may make clear that the setting is a library and thus the person writing or speaking the above sentence may choose to omit that information, while maintaining grammatical correctness, with a listener being able to discern the semantically complete version.

It's possible when parsing sentences into a semantic tree representation, as above, that some argument slots may be blank. These slots can be filled by either nouns or semantic subtrees and empty slots can help a natural language understanding system to know what is unknown when processing a document.

Some NLU systems may be able to discern ranked candidates for these from previous content or context. Algorithmically, this can be achieved by maintaining a context state during document processing or utilizing an event driven knowledge acquisition engine.

If there's anything to this sentence-level semantic abbreviation in natural speech and writing, then NLG systems might be able to utilize it to produce less mechanical sounding text; a sequence of semantically complete sentences might sound formal or verbose.