code: def coefficient_between_units(unit_a, unit_b):
      
      for mapping in unit_mapping:
          if unit_a == mapping[0] and unit_b == mapping[1]:
              return mapping[2]
          if unit_a == mapping[1] and unit_b == mapping[0]:
              return 1 / mapping[2]
  
      return None
docs: A helper to get the coefficient between two units.
  
      :param unit_a: The first unit.
      :type unit_a: safe.definitions.units
  
      :param unit_b: The second unit.
      :type unit_b: safe.definitions.units
  
      :return: The coefficient between these two units.
      :rtype: float