function SST = strrep_sst(s1,s2,ALF)
% creates a subsequential transducer that replaces 
% all occurrences of str s1 with str s2

% these have no accept states

n = length(s1);

SST.ALF = ALF;
SST.Q = 1:n;
SST.q0 = 1;

for i=1:n
  for sig=ALF
    if (sig==s1(i))
      if (i==n)
	% we've read in the whole s1 -- emit s2 and return to state 1
	SST = set_sstd(SST,n,sig,1,s2);
      else
	% we're somewhere in the middle of s2 -- emit nothing for now
	SST = set_sstd(SST,i,sig,i+1,'');
      end
    else
      SST = set_sstd(SST,i,sig,1,[s1(1:i-1) sig]);
    end
  end
end

return


function junk
sigs = setstar(ALF);
MM = null_dfsa(ALF);
for j=1:length(s1)
  u = s1(1:j);
  reg = [sigs u ];
  M = gregexp2dfsa_fast(reg,ALF);
  MM = union_dfsa(MM,M);
end

% accept states of Mu emit s2
return