new_contents: import numpy as np
  
  from molml.features import Connectivity
  from molml.constants import BOND_LENGTHS
  
  # Currently, there are two recommended ways to work with elements that are not
  # included in molml/constants.py. In this example, we will look at an iron
  # complex (iron is not in the constants).
  # Maybe at some point, molml will include more constants, but it seems outside
  # of the scope of this library.
  
  if __name__ == '__main__':
      elements = ['Fe', 'H', 'H', 'H', 'H', 'H', 'H']
      coords = np.array([
          [0., 0., 0.],
          [1.46, 0., 0.],
          [0., 1.46, 0.],
          [0., 0., 1.46],
          [-1.46, 0., 0.],
          [0., -1.46, 0.],
          [0., 0., -1.46],
      ])
      feat = Connectivity(depth=2)
      # Notice the warning about missing elements.
      print(feat.fit_transform([(elements, coords)]))
  
      # 1) Modify the values in the constants module before your script.
      BOND_LENGTHS['Fe'] = {'1': 1.32}
      print(feat.fit_transform([(elements, coords)]))
      del BOND_LENGTHS['Fe']
  
      # 2) Include connectivity information in your data. The other instances
      # where constants are used (electronegativity, element symbols, atomic
      # numbers).
      connections = {
          0: {1: '1', 2: '1', 3: '1', 4: '1', 5: '1', 6: '1'},
          1: {0: '1'},
          2: {0: '1'},
          3: {0: '1'},
          4: {0: '1'},
          5: {0: '1'},
          6: {0: '1'},
      }
      print(feat.fit_transform([(elements, coords, connections)]))
  
subject: Add an example script with missing constants