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/usr/share/perl5/Mail/IMAPClient/Thread.grammar is in libmail-imapclient-perl 3.39-1.

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# Atoms:

NUMBER: /\d+/

# Rules:

threadmember:	NUMBER    { $return = $item{NUMBER} ; }	|
		thread 	  { $return = $item{thread} ; } 

thread:	"(" threadmember(s) ")"
	{
		$return = $item{'threadmember(s)'}||undef;	
	}

# Start:
start: /^\* THREAD /i thread(s?) {
	$return=$item{'thread(s?)'}||undef;
}