docs: def COH(self):
          """Coherence.
  
          .. math:: \mathrm{COH}_{ij}(f) = \\frac{S_{ij}(f)}
                                                 {\sqrt{S_{ii}(f) S_{jj}(f)}}
  
          References
          ----------
          P. L. Nunez, R. Srinivasan, A. F. Westdorp, R. S. Wijesinghe,
          D. M. Tucker, R. B. Silverstein, P. J. Cadusch. EEG coherency I:
          statistics, reference electrode, volume conduction, Laplacians,
          cortical imaging, and interpretation at multiple scales. Electroenceph.
          Clin. Neurophysiol. 103(5): 499-515, 1997.
          """
          S = self.S()
          # TODO: can we do that more efficiently?
          return S / np.sqrt(np.einsum('ii..., jj... ->ij...', S, S.conj()))