Right, I couldn't find any proven source for that.
1	Right	right	INTJ	UH	_	6	discourse	6:discourse	SpaceAfter=No
2	,	,	PUNCT	,	_	6	punct	6:punct	_
3	I	I	PRON	PRP	Case=Nom|Number=Sing|Person=1|PronType=Prs	6	nsubj	6:nsubj	_
4-5	couldn't	_	_	_	_	_	_	_	_
4	could	could	AUX	MD	VerbForm=Fin	6	aux	6:aux	_
5	n't	not	PART	RB	_	6	advmod	6:advmod	_
6	find	find	VERB	VB	VerbForm=Inf	0	root	0:root	_
7	any	any	DET	DT	_	9	det	9:det	_
8	proven	prove	VERB	VBN	Tense=Past|VerbForm=Part	9	amod	9:amod	_
9	source	source	NOUN	NN	Number=Sing	6	obj	6:obj	_
10	for	for	ADP	IN	_	11	case	11:case	_
11	that	that	PRON	DT	Number=Sing|PronType=Dem	9	nmod	9:nmod:for	SpaceAfter=No
12	.	.	PUNCT	.	_	6	punct	6:punct	_
