docs: def convert_atomic_to_mass_fractions(atomic_fractions):
      """
      Converts an atomic fraction :class:`dict` to a mass fraction :class:`dict`.
  
      Args:
          atomic_fractions (dict): atomic fraction :class:`dict`.
              The composition is specified by a dictionary.
              The keys are atomic numbers and the values atomic fractions.
              No wildcard are accepted.
      """
      # Calculate total atomic mass
      atomic_masses = {}
      total_atomic_mass = 0.0
      for z, atomic_fraction in atomic_fractions.items():
          atomic_mass = pyxray.element_atomic_weight(z)
          atomic_masses[z] = atomic_mass
          total_atomic_mass += atomic_fraction * atomic_mass
  
      # Create mass fraction
      mass_fractions = {}
      for z, atomic_fraction in atomic_fractions.items():
          mass_fractions[z] = atomic_fraction * atomic_masses[z] / total_atomic_mass
  
      return mass_fractions