Two-Stage Constraint Based Hindi Parser

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Two-Stage Constraint Based Hindi Parser
LTRC, IIIT Hyderabad
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Brief Recap

• Broad coverage parser
• Dependency
• Paninian framework
• vibhakti-karaka correspondence
• karaka frames (basic transformation)
• Source groups, demand groups
• Constraints
• Three basic constraints
• Constraints as Integer programming equations

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Parser

• Two stage strategy
• Appropriate constraints formed
• Stage I (Intra-clausal relations)
• Dependency relations marked
• Relations such as k1, k2, k3, etc. for each verb
• Stage II (Inter-clausal relations conjunct
  relations)
• Conjuncts, relative clauses, kriya mula, etc
• In certain cases, separates syntax from semantics
  (eg. kriya mula), in others, reduces the
  complexity.

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Steps in Parsing
SENTENCE
Morph, POS tagging, Chunking
Identify Demand Groups
STAGE - II
Load Frames Transform
YES
Is Complex
NO
Find Candidates
Apply Constraints Solve
Final Parse
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Stage I Types being handled

• Simple Sentences (finite verbs)
• Clausal arguments
• Non-finite verbs
• wA_huA
• wA_hI
• nA
• kara
• 0_rahe, etc.
• Copula
• Genitive

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Stage - II

• Handles
• Conjuncts
• Subordinating Coordinating
• Relative clauses
• Complex predicates
• Basic constraints similar to Stage-I
• Some additional constraints
• New demand groups
• New candidates

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Steps (Stage II)
Identify New Demand Groups
Load Frames Transform
Output of STAGE - I
Find Candidates
Repair
Apply Constraints Solve
FINAL PARSE
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Example Relative Clause

• vaha puswaka jo rAma ne mohana ko
  xI hE prasixXa hE
• that book which Ram ERG. Mohana
  DAT. gave is famous is
• The book which Ram gave to Mohana is famous

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Output after Stage - I
_ROOT_
main
main
hE
xI
k1
k1s
prasixXa
puswaka
k2
k1
k4
vaha
jo
mohana
rAma
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Identify the demand group

• xiyA give
• Main verb of the relative clause

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Identify the demand group,Load and Transform DF

• jo which transformation (special)
• Transforms the demand frame of the main verb of
  the relative clause
• --------------------------------------------------
  --------------------------------------------------
  ----------
• arc-label necessity vibhakti
  lextype src-pos arc-dir
  oprt
• --------------------------------------------------
  --------------------------------------------------
  ----------
• nmod__relc m any
  n rl p
  insert
• --------------------------------------------------
  --------------------------------------------------
  ----------

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Karaka Frame
Main verb of relative clause

• vaha puswaka jo rAma ne mohana
  ko xI prasixXa hE
• that book which Ram ERG. Mohana
  DAT. gave famous is
• The book which Ram gave to Mohana is famous

Transformed frame for xe after applying the jo
trasformation
--------------------------------------------------
--------------------------------------------------
---- arc-label necessity vibhakti
lextype src-pos arc-dir
oprt ---------------------------------------------
--------------------------------------------------
--------- nmod__relc m any
n rl
p insert --------------------------------
--------------------------------------------------
-----------------------
New row inserted after transformation
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Possible candidates
nmod__relc

• vaha puswaka jo rAma ne mohana ko xI hE
  prasixXa hE

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Output after Stage - II
_ROOT_
main
hE
k1
k1s
prasixXa
vaha puswaka
nmod__relc
xiyA hE
k1
k2
k4
rAma
mohana
jo
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Example II Coordination

• rAma Ora siwA kala Aye
• Ram and Sita yesterday came
• Ram and Sita came yesterday

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Output of Stage - I
_ROOT_
dummy
main
dummy
rAma
Aye
Ora
k1
k7t
siwA
kala
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For Stage II (Constraint Graph)
_ROOT_
main
rAma
Aye
Ora
k1
k7t
ccof
siwA
ccof
kala
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Candidate Arcs
_ROOT_
main
k1
rAma
Aye
Ora
k1
k1
ccof
siwA
ccof
kala
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Solution Graph
_ROOT_
main
k1
rAma
Aye
Ora
k7t
ccof
siwA
ccof
kala
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Parse tree
_ROOT_
main
Aye
k7t
k1
kala
Ora
ccof
ccof
siwA
rAma
Output after Stage II
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Finite Verb Coordination

• rAma Gara gayA Ora vaha so gayA
• Ram home went and he
  sleep went
• Ram went home and slept

_ROOT_
main
main
dummy
so
gayA
Ora
k1
k1
k2
vaha
rAma
Gara
Output after Stage I
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Karaka Frame - Ora
Finite
Ora
Ora
ccof
ccof
ccof
ccof
v_fin
gayA
v_fin
so
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Finite Verb Coordination (Parse Tree)
_ROOT_
main
Ora
ccof
ccof
gayA
so
k1
k1
k2
rAma
vaha
Gara
Output after Stage II
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Relative Clause Coordination

• rAma ne vaha puswaka KarIxI jo prasixXa hE Ora
  jo saswI hE
• Ram purchased the book which is famous and
  which is cheap

_ROOT_
main
main
main
dummy
KarIxI
hE
Ora
hE
k1
k1s
k1
k1s
k2
k1
jo
prasixXa
jo
saswI
puswaka
rAma
Output after Stage I
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Karaka Frame - Ora
Relative Clause
n
puswaka
nmod__relc
nmod__relc
Ora
Ora
ccof
ccof
ccof
ccof
v_rel
v_rel
hE
hE
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Relative Clause Coordination (Parse Tree)
_ROOT_
main
KarIxI
k2
k1
puswaka
rAma
nmod__relc
Ora
ccof
ccof
hE
hE
k1
k1s
k1
k1s
jo
prasixXa
jo
saswI
Output after Stage II
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Non-Finite Verb Coordination

• rAma Kelakara Ora KAnA KAkara so
  gayA
• Ram having played and food having
  eaten sleep went

_ROOT_
main
dummy
so
Ora
vmod
vmod
k1
rAma
Kelakara
KAkara
k2
KAnA
Output after Stage I
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Karaka Frame - Ora
Non-Finite
so
v_fin
Ora
Ora
ccof
ccof
ccof
ccof
v_nfin
v_nfin
Kelakara
KAkara
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Non-Finite Verb Coordination (Parse Tree)
_ROOT_
main
so
vmod
k1
Ora
rAma
ccof
ccof
KAkara
Kelakara
k2
KAnA
Output after Stage II
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Nominal Coordination

• rAma Ora siwA kala Aye
• Ram and Sita yesterday
  came
• Ram and Sita came yesterday

_ROOT_
dummy
main
dummy
rAma
Aye
Ora
k1
k7t
siwA
kala
Output after Stage I
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Karaka Frame - Ora
Nominal
Ora
Ora
ccof
ccof
ccof
ccof
siwA
rAma
n
n
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Nominal Coordination (Parse Tree)
_ROOT_
main
Aye
k7t
k1
kala
Ora
ccof
ccof
siwA
rAma
Output after Stage II
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Example

• rAma Ora siwA kala Aye

_ROOT_
dummy
main
dummy
rAma
Aye
Ora
k1
k7t
siwA
kala
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Steps (Stage II)
Identify Nodes
Load Frames Transform
Identify New Demand Groups
Output of STAGE - I
Find Candidates
Repair
Apply Constraints Solve
FINAL PARSE
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Constraint Graph Nodes (Stage II)

• Selected from the intermediate parse tree (Stage
  I)
• Set-I (demand nodes)
• Conjuncts
• Nearest verbal ancestor of jo (usually just the
  parent)
• _ROOT_
• Children of _ROOT_ other than (1) and (2).
• Other nodes which are added due to nodes in Set 2

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Constraint Graph Nodes (Stage II)

• Set-II (source nodes)
• Possible children and parents of conjuncts
• Possible heads of the relative clause.
• Identification of nodes in Set-II will generally
  trigger the repair.

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Steps (Stage II)
Identify Nodes
Load Frames Transform
Identify New Demand Groups
Output of STAGE - I
Find Candidates
Repair
Apply Constraints Solve
FINAL PARSE
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Identify the demand group

• Ora
• Aye

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Steps (Stage II)
Identify Nodes
Load Frames Transform
Identify New Demand Groups
Output of STAGE - I
Find Candidates
Repair
Apply Constraints Solve
FINAL PARSE
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General Principles

• Repair/Revision
• Any node which becomes a potential child in stage
  2, its arc to its existing parent is open to
  revision
• rAma Ora siwA kala Aye

• Node 4 becomes potential child (of node
• 1)
• Its parent (node 2) is open to revision

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General Principles

• Repair/Revision after parse of stage I
• Any node which becomes a potential parent must be
  re-looked at.
• rAma Ora siwA kala Aye

• Node 2 becomes potential parent (of 1)
• Its child (node 4) is open to revision

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Algorithm

• Identify nodes of the constraint graph
• From Set 1, and
• From Set 2
• Remove all outgoing edges from _ROOT_.
• Find possible candidates for demand nodes present
  in Set 1 from Set 2
• Parent candidate for finite verb
• Parent and children for conjuncts
• Children of _ROOT_
• Convert the formed constraint graph into integer
  programming (IP) problem.
• Solve the IP equations to get the possible
  solution parse.

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An example

• raama aura sitaa kala aaye
• Ram and Sita yesterday
  came
• Ram and Sita came yesterday

_ROOT_
dummy
main
dummy

• Output after stage I

rAma
Aye
Ora
k1
k7t
siwA
kala
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Identify Nodes
_ROOT_
dummy

• Set 1 nodes

main
dummy
rAma
Aye
Ora
k1
k7t
siwA
kala
_ROOT_
dummy

• Set 1 and Set 2

main
dummy
rAma
Aye
Ora
k1
k7t
siwA
kala
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Constraint Graph

• New Constraint Graph
• Ora, Aye and _ROOT_ are the demand groups
• Note kala remains attached to its parent
  aaye (does not show up in stage 2)

_ROOT_
main
k1
ccof
Aye
Ora
rAma
k1
ccof
siwA
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Example
_ROOT_
main

• Final Parse

Aye
k7t
k1
kala
Ora
ccof
ccof
siwA
rAma
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Types of complex sentences

• Relative clauses
• Initial
• Final
• Medial
• Conjuncts (Coordination)
• Simple clause
• Relative clause
• Non-finite
• Nominal, adjectival, adverbial

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Some other examples

• rAma ne vaha puswaka KarIxI jo saswI hE Ora jo
  bAjZAra meM prasixXa hE
• samIra Ora aBay ne vaha puswaka KarIxI jo saswI
  hE Ora jo bAjZAra meM prasixXa hE
• rAma Ora mohana ke xoswa kI baccI Aye
• Only baccI came, or
• Both rAma and baccI came
• Use of gnp of the main verb, Aye vs. AI

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• THANKS!!