What Stillman did on these walks remained something of a mystery to Quinn.
1	What	what	PRON	WH-REL	PronType=Rel	3	obj	_	_
2	Stillman	Stillman	PROPN	SG-NOM	Number=Sing	3	nsubj	_	_
3	did	do	AUX	PAST-AUX	Mood=Ind|Tense=Past|VerbForm=Fin	7	csubj	_	_
4	on	on	ADP	_	_	6	case	_	_
5	these	this	DET	DEM-PL	Number=Plur|PronType=Dem	6	det	_	_
6	walks	walk	NOUN	PL-NOM	Case=Nom	3	obl	_	_
7	remained	remain	VERB	PAST	Mood=Ind|Tense=Past|VerbForm=Fin	0	root	_	_
8	something	something	PRON	IND-SG-NOM	Number=Sing	7	obj	_	_
9	of	of	ADP	_	_	11	case	_	_
10	a	a	DET	IND-SG	Definite=Ind|PronType=Art	11	det	_	_
11	mystery	mystery	NOUN	SG-NOM	Number=Sing	8	nmod	_	_
12	to	to	ADP	_	_	13	case	_	_
13	Quinn	Quinn	PROPN	SG-NOM	Number=Sing	7	obl	_	SpaceAfter=No
14	.	.	PUNCT	Period	_	7	punct	_	_
