nulls in propbank sep 17 th 2010. what is a null category

Nulls in PropBank

Sep 17th 2010

What is a null category

PropBank annotation

• Shyam ne khana khaaya• Annotate Arguments of the verb `Khaa’• Shyam and khana

• What if someone asks- kisne khaana khaaya?• Shyam ne khaaya• Shyam ne *khaana* khaaya (Object dropped)

• What about: tumne Ram ko kaunsi kitaab di?• Nili kitaab raam ko di• *maine* nili kitaab raam ko di (Subject dropped)

Missing information!

• Same verb, but varying number of arguments:– Ram ne khaana khaaya– Ram ne *khaana* khaaya

• No consistency in the representation of the verb `khaana’

• Hence, we need to insert missing arguments, or as they are also known ‘Empty Categories (EC)’

Which missing arguments?

• Only the obligatory arguments of the verb• Khaa requires two arguments- eater and thing eaten• When we ask a question: Ram ne kya khaaya?• It becomes possible to drop Ram & respond by saying:

khaana khaaya• Ram is old information and is not repeated

• If obligatory arguments are missing, the correct argument structure of a verb will not be understood by the machine

That’s great!

Four categories of EC

• pro• PRO• GAP-pro• RELPRO

pro

• Missing subject and object e.g responding to a question– *pro* mujhe hari ko denaa hai– Kisne darvaaza kholaa? Mohan ne *pro* kholaa

• Dropped subject in a sentence– Bacchon ke liye karyakram aayojan karna hogaa

pro characteristics

• Fewer surface cues• Requires knowledge of the verb’s obligatory

arguments

pro is the missing VIP!

PRO

• When a sentence has an embedded clause with a non-finite verb AND the same (missing) subject as the main clause, we get *PRO* – Note two important points:– Mohani ne [*PRO*i kitaab paRnii] caahi• [Khaana khaana] has the verb `khaana’ in non-finite

form i.e it’s not showing agreement or tense info• We coindex the PRO with Mohan• There are exceptions!!

For PRO cases, the verb is non-finite, and handicapped!

Exceptions to PRO

• Not present for the modals e.g– Raam ko ghar jaana paDaa / caahiye – Raam ko ghar jaane diyaa

• Verbs with lagnaa (more of a helping verb)– Raam naachne lagaa

• Not in location information type sentences– Ram ghar jaane se pehle vahan rukaa

PRO characteristics

• भा�रत सर्का��र ने [ऐस� र्कारने स] इं र्का�र र्कार दि�या�

The verb ends with ने�, ने�, ने It has a corresponding argument to coindex with! If you replace it with an actual word, it sounds

ungrammatical

GAP PRO

• Only in co-ordinate clauses!!!• Clauses joined by और, या� or a comma

– mohan-nei kitaab paRh-ii aur [*GAP-pro*]i so ga-yaa

– Note that we only look for missing arguments in co-ordination structures

– We also coindex like the *PRO*

GAP-PRO only found with co-ordination!

Context for RELPRO

(1) phool jo daaliyon par khile hue hai, ache hai

• (1) is an example of a relative clause– phool is the noun that’s getting modified– In a relative clause, there’s always an embedded

‘modifier’ clause inside the main clause– phool [jo daaliyon par khile hue hai], ache hai

Upside down clause

• In the case of participial relatives, the order of elements in the relative clause is flipped

• [daaliyon par khile] phool ache hai• The embedded clause is before the element being

modified• Also, there is no relative pronoun ‘jo’ like we can see in (1)

• In such conditions, we will insert a missing subject argument in the upside down clause

• (2) [*RELPRO*daaliyon par khile hue] phool ache hai

RELPRO characteristics

• इंसस [तटों� पर रहने वा�ले] जि�वा � त�ओं र्का� भा� ने�र्कास�ने ह�गा�

Participial verbs can be identified by -त� हुआ , -ने वा�ले e.g र्कारत� हुआ, र्कारने वा�ले

They also have the drel nmod__k1inv or nmod__k2inv

RELPRO: the upside down EC

More practice!