Archived discussion on Ellipsis in UD v2
This is an archive page. The final documentation is here: http://universaldependencies.org/u/overview/specific-syntax.html#ellipsis
remnant relation turned out to be a non-optimal way for analyzing complex ellipitical constructions. We therefore introduce the following changes in UD v2:
remnantrelation is discarded
- In the case of simple head ellipsis, a dependent is promoted
- In case of predicate ellipsis, we also use promotion but introduce a new relation
orphanfor non-standard dependency relations that arise
- The complex cases of ellipsis should be analyzed with NULL nodes in the enhanced representation
Problems with the
The old analysis of elliptical constructions using the
remnant relation does not work well when the second clause contains additional modifiers of the elided predicate.
They had left the company , many for good . nsubj(left, They) obj(left, company) remnant(They, many) obl(many, good)
In this example, for good is modifying the elided left of the second clause. However, as no similar modifier exists in the first clause, for good cannot be attached with a remnant relation and no reasonable analysis of this sentence is possible. In practice, annotators attached the extra modifier to the subject of the second clause, which incorrectly suggests that for good is modifying the subject many.
Additional issues of the
remnant analysis are:
- The dependency trees contain a lot of non-projective dependencies, which complicates parsing.
- The requirement that every argument/modifier of the elided predicate has an antecedent, makes it impossible to analyze cross-sentence elliptical constructions. (This would require relations crossing sentence boundaries.)
Head ellipsis in UD v2
In the following cases of head ellipsis, we promote a dependent of the elided head.
If the head nominal is elided, we promote dependents in the following order:
Er kauft sich ein grünes Auto und sie kauft sich ein rotes . \n He buys himself a green car and she buys herself a red . nsubj(kauft-2, Er-1) det(Auto-6, ein-4) amod(Auto-6, grünes-5) obj(kauft-2, Auto-6) conj(kauft-2, kauft-9) nsubj(kauft-9, sie-8) obj(kauft-9, rotes-12) det(rotes-12, ein-11)
She saw every animal at the zoo but he saw only some . nsubj(saw-2, She-1) det(animal-4, every-3) obj(saw-2, animal-4) conj(saw-2, saw-10) advmod(some-12, only-11) obj(saw-10, some-12)
She saw three monkeys and he saw two . nsubj(saw-2, She-1) nummod(monkeys-4, three-3) obj(saw-2, monkeys-4) conj(saw-2, saw-7) obj(saw-7, two-8)
If the main predicate is elided, we use simple promotion only if there is an
cop, or a
mark in the case of an infinitival marker.
Sue likes pasta and Peter does , too . nsubj(likes-2, Sue-1) obj(likes-2, pasta-3) conj(likes-2, does-6) nsubj(does-6, Peter-5) advmod(does-6, too-8)
Sue is hungry and Peter is , too . nsubj(hungry-3, Sue-1) cop(hungry-3, is-2) conj(hungry-3, is-6) nsubj(is-6, Peter-5) advmod(is-6, too-8)
They will do it if they want to . nsubj(will-2, They-1) aux(do-3, will-2) obj(it-4, do-3) advcl(do-3, want-7) nsubj(want-7, they-6) xcomp(want-7, to-8)
Predicate ellipsis in Basic UD v2
In more complicated cases where a predicate is elided but no
cop is present, promotion would lead to very unnatural and confusing relations. For example, in the following sentence, you would be the subject of coffee, suggesting that the second clause contains a copular construction rather than an elided predicate.
I like tea and you coffee . nsubj(like-2, I-1) obj(like-2, tea-3) nsubj(coffee-6, you-5) conj(like-2, coffee-6)
We considered two alternative proposals for dealing with such cases in UD basic dependencies, one that make use of composite relations (but do not introduce any new relations) and one that instead adds a new relation named
orphan to preserve intuitions about constituency. In the end we adopted alternative 2.
Predicate ellipis 1: composite relations (rejected variant)
The first alternative is to attach orphans to their grandparent with a composite relation of the form
I like tea and you coffee . nsubj(like-2, I-1) obj(like-2, tea-3) conj>nsubj(like-2, you-5) conj>obj(like-2, coffee-6)
If the grandparent is also elided, the relation is composed of all three relations and the orphan is attached to its great-grandparent.
Mary wants to buy a book and Jenny a CD . nsubj(wants-2, Mary-1) xcomp(wants-2, buy-4) obj(buy-4, book-6) conj>nsubj(wants-2, Jenny-8) conj>xcomp>obj(wants-2, CD-10)
Unlike the analysis using the
remnant relation, this proposal also allows us to analyze sentences in which the second clause contains additional modifiers.
They had left the company , many for good . nsubj(left, They) obj(left, company) conj>nsubj(left, many) conj>nmod(left, good)
This approach can also be used when the antecedent of the elided node is in another sentence. The artificial ROOT node is now allowed to have more than one child, but only if all are attached via composite relations, starting with
Mary wants to buy a book . ROOT And Jenny a CD . nsubj(wants-2, Mary-1) xcomp(wants-2, buy-4) obj(buy-4, book-6) root>nsubj(ROOT, Jenny) root>xcomp>obj(ROOT, CD)
Predicate ellipis 2: orphan instead of remnant (approved variant)
The second alternative preserves the integrity of the second conjunct as a single subtree by (arbitrarily) promoting one of the orphans to the (subclause) root and attaching the others with a new dummy relation
orphan. Here are the same examples annotated according to this alternative:
I like tea and you coffee . nsubj(like-2, I-1) obj(like-2, tea-3) conj(like-2, you-5) cc(you-5, and-4) orphan(you-5, coffee-6)
Mary wants to buy a book and Jenny a CD . nsubj(wants-2, Mary-1) xcomp(wants-2, buy-4) obj(buy-4, book-6) conj(wants-2, Jenny-8) orphan(Jenny-8, CD-10)
They had left the company , many for good . nsubj(left, They) obj(left, company) conj(left, many) orphan(many, good)
Mary wants to buy a book . ROOT And Jenny a CD . nsubj(wants-2, Mary-1) xcomp(wants-2, buy-4) obj(buy-4, book-6) root(ROOT, Jenny) orphan(Jenny, CD)
Note that the
orphan relation is only used when an ordinary relation would be misleading (for example, when attaching an object to a subject). In particular, the ordinary
cc relation should be used for the coordinating conjunction, which attaches to the pseudo-constituent formed through the
All things considered, alternative 2 was judged to be the best analysis because it preserves the integrity of clauses, avoids the introduction of complex labels, and harmonizes well with the promotion analysis used for simpler cases of ellipsis.
Predicate ellipsis in Enhanced UD v2
While we hold on to the principle that basic UD trees have to be strict surface syntax trees, we propose to relax this requirement in the enhanced representation and to allow special null nodes for sentences with elided predicates. These nodes have special word indices of the form Ea.b, where a is the index of the token that would precede the elided word and b is a counter. (See also the description of the changes to the CoNLL-U file format.) Whenever the basic representation contains an instance of the
orphan relation, the enhanced representation contains additional null nodes so that all orphans can be attached to their real (ellided) parent.
For example, the sentences from the previous section are analyzed as following in the enhanced representation. (The special null nodes are labelled with Ea.b .)
I like tea and you E5.1 coffee . nsubj(like-2, I-1) obj(like-2, tea-3) nsubj(E5.1-6, you-5) conj(like-2, E5.1-6) obj(E5.1-6, coffee-7)
Mary wants to buy a book and Jenny E8.1 E8.2 a CD . nsubj(wants-2, Mary-1) xcomp(wants-2, buy-4) obj(buy-4, book-6) conj(wants-2, E8.1-9) nsubj(E8.1-9, Jenny-8) xcomp(E8.1-9, E8.2-10) obj(E8.2-10, CD-12)
They had left the company , many E7.1 for good . nsubj(left, They) obj(left, company) conj(left, E7.1) nsubj(E7.1, many) nmod(E7.1, good)
In the first example, the node E5.1 is added for the elided predicate like. In the second example, we add one node for the elided matrix verb wants (E8.1) and one node for the elided embedded verb buy (E8.2). As the elided marker to does not have any dependents, we do not add a null node for it.