def make_prev_next(seg_table):
	"""
	Function to make two column table into a four column table with prev/next seg
	:param seg_table: Input table of form:

			They	They
			don't	do|n't
			know	know

	:return: Four column table with prev/next group context columns:

			_       don't   They    They
			They    know    don't   do|n't
			don't	_       know	know
	"""

	prev_group = "_"
	segs = [tuple(i.split('\t')) for i in seg_table]
	out_segs = []
	for i, line in enumerate(segs):
		current_group, segmentation = line
		if i < len(seg_table) - 1:
			next_group = segs[i+1][0]
		else:
			next_group = "_"  # Last group in data
		if i > 0:
			prev_group = segs[i-1][0]
		out_segs.append("\t".join([prev_group, next_group, current_group, segmentation]))

	return out_segs