function index = look_for_val(desc, field_name_1, field_name_2, val, ...			      stop_on_error)  index = [];  f1 = getfield(desc, field_name_1);  for ii = 1:length(f1)    f2 = getfield(f1(ii), field_name_2);    if strcmp(f2, val)      index = ii;      break    end  end  if stop_on_error    if isempty(index)      error([field_name_1 '.' field_name_2 ' does not contain ' val])    end  end