function s = sample_from_sp(R,lam)
% larger lam => longer strings
s = '';
for i=1:length(R)
  A = R{i};
  if length(A)==1
       s(end+1) = A;
  else
    while 1
      a = randel(A);
      if a == '@'
        if rand > lam
          break
        end
      else
        s(end+1) = a;
      end
    end
  end
end
s = strrep(s,'@','');
return