function sig = lexordtree(T)
% T is a directed tree
% all edges (i,j) are of the form i<j

[n,n] = size(T);
sig = zeros(1,n);

% first find the root.
% the root is the unique node that 
% nobody points to

fanin = sum(T);
ii = find(~fanin);
if length(ii)~=1
  error('no unique root node');
end

sig = [];
Q = ii;

% now do a breadth-first traversal
while ~isempty(Q)
  % pop node off Q
  node = Q(end);
  Q = Q(1:end-1);
  
  sig = [sig node];
  % get the nodes children
  kids = find(T(node,:));
  
  Q = [kids Q];
end

return