function edge = findEdge(G, mask)
% part of eulerWalk. Find an edge adjacent to a node in the mask.

if (length(mask)==0),
	mask = 1:size(G,1);
end

for i=1:length(mask),
	node = mask(i);
	nbrs = G(node,:);
	nbrs = nbrs(nbrs>0);
	if (length(nbrs)>0),
		edge = [node; nbrs(1)];
		return;
	end
end

error('can''t find edge!');