In other words, start and end tag must be properly nested within other elements.
1	In	in	ADP	_	_	3	case	_	_
2	other	other	ADJ	POS	Degree=Pos	3	amod	_	_
3	words	word	NOUN	PL-NOM	Number=Plur	12	nmod	_	SpaceAfter=No
4	,	,	PUNCT	Comma	_	3	punct	_	_
5	start	start	NOUN	SG-NOM	Number=Sing	8	compound	_	_
6	and	and	CCONJ	_	_	7	cc	_	_
7	end	end	NOUN	SG-NOM	Number=Sing	5	conj	_	_
8	tag	tag	NOUN	SG-NOM	Number=Sing	12	nsubj	_	_
9	must	must	AUX	PRES-AUX	VerbForm=Fin	12	aux	_	_
10	be	be	AUX	INF	VerbForm=Inf	12	cop	_	_
11	properly	properly	ADV	_	_	12	advmod	_	_
12	nested	nested	ADJ	POS	Degree=Pos	0	root	_	_
13	within	within	ADP	_	_	15	case	_	_
14	other	other	ADJ	POS	Degree=Pos	15	amod	_	_
15	elements	element	NOUN	PL-NOM	Number=Plur	12	obl	_	SpaceAfter=No
16	.	.	PUNCT	Period	_	12	punct	_	_
