“…It has recently proven to be a powerful tool for inverse spectral theory for these operators and further refinements were given in [3], [4], [5], [6], [7], [10], [20]. The analogous theory for one-dimensional Dirac operators was developed by Brunnhuber and three of us in [2] (for further extensions see also [8], [9]). Nevertheless, such operators are still difficult to understand.…”