docs: def valid_backbone_bond_lengths(self, atol=0.1):
          """True if all backbone bonds are within atol Angstroms of the expected distance.
  
          Notes
          -----
          Ideal bond lengths taken from [1].
  
          References
          ----------
          .. [1] Schulz, G. E, and R. Heiner Schirmer. Principles Of
             Protein Structure. New York: Springer-Verlag, 1979.
  
          Parameters
          ----------
          atol : float, optional
              Tolerance value in Angstoms for the absolute deviation
              away from ideal backbone bond lengths.
          """
          bond_lengths = self.backbone_bond_lengths
          a1 = numpy.allclose(bond_lengths['n_ca'],
                              [ideal_backbone_bond_lengths['n_ca']] * len(self),
                              atol=atol)
          a2 = numpy.allclose(bond_lengths['ca_c'],
                              [ideal_backbone_bond_lengths['ca_c']] * len(self),
                              atol=atol)
          a3 = numpy.allclose(bond_lengths['c_o'],
                              [ideal_backbone_bond_lengths['c_o']] * len(self),
                              atol=atol)
          a4 = numpy.allclose(bond_lengths['c_n'],
                              [ideal_backbone_bond_lengths['c_n']] *
                              (len(self) - 1),
                              atol=atol)
          return all([a1, a2, a3, a4])