function c2tbn = canonize( dag, ss, gnames )
% ss marks the halfway mark between the two time slices
% names supplied in num2name.m

keepers = zeros(1,ss);
for i=1:ss,
  % keep all the t-1 nodes that have a link to t slice
  for j=(ss+1):(2*ss),
    if dag(i,j) == 1,
      keepers(i) = 1;
      break;
    end;
  end;
end;
% checking we didn't keep/avoid more
for i=1:ss,
  if keepers(i) == 1,
    if nargin < 3, 
      disp(num2name(i));
    else
      disp(gnames(i));
    end;
  end;
end;
c2tbn = keepers;
% start deleting non-keepers
sizeofc2tbn = length(find(keepers))+ss;
pos_2ndslice = ss+1;
c2tbn = dag;
while 1,
  if length(c2tbn) <= sizeofc2tbn,
    break;
  else
    % if node doesn't have temporal link then delete it
    for i=1:length(c2tbn),
      node = i;
      found = 0;
      for j=pos_2ndslice:length(c2tbn),
        if c2tbn(i,j) == 1, 
          found = 1;
          break;
        end;
      end;
      if found == 0,
        c2tbn = rmnode( c2tbn, node );
        pos_2ndslice = pos_2ndslice - 1;
        break;
      end;
    end;
  end;
end;