Although many theoretical and empirical models have been developed, some problems are still unsolved.
1	Although	although	SCONJ	IN	_	9	mark	9:mark	Discourse=adversative-concession:89->90:0
2	many	many	ADJ	JJ	Degree=Pos	6	amod	6:amod	Entity=(172-abstract-new-cf1-5-sgl
3	theoretical	theoretical	ADJ	JJ	Degree=Pos	6	amod	6:amod	_
4	and	and	CCONJ	CC	_	5	cc	5:cc	_
5	empirical	empirical	ADJ	JJ	Degree=Pos	3	conj	3:conj:and|6:amod	_
6	models	model	NOUN	NNS	Number=Plur	9	nsubj:pass	9:nsubj:pass	Entity=172)
7	have	have	AUX	VBP	Mood=Ind|Number=Plur|Person=3|Tense=Pres|VerbForm=Fin	9	aux	9:aux	_
8	been	be	AUX	VBN	Tense=Past|VerbForm=Part	9	aux:pass	9:aux:pass	_
9	developed	develop	VERB	VBN	Tense=Past|VerbForm=Part|Voice=Pass	15	advcl	15:advcl:although	SpaceAfter=No
10	,	,	PUNCT	,	_	9	punct	9:punct	_
11	some	some	DET	DT	PronType=Ind	12	det	12:det	Discourse=organization-preparation:90->96:3|Entity=(173-abstract-new-cf2-2-sgl
12	problems	problem	NOUN	NNS	Number=Plur	15	nsubj	15:nsubj	Entity=173)
13	are	be	AUX	VBP	Mood=Ind|Number=Plur|Person=3|Tense=Pres|VerbForm=Fin	15	cop	15:cop	_
14	still	still	ADV	RB	Degree=Pos	15	advmod	15:advmod	_
15	unsolved	unsolved	ADJ	JJ	Degree=Pos|Polarity=Neg	0	root	0:root	SpaceAfter=No
16	.	.	PUNCT	.	_	15	punct	15:punct	_
