code: def dlf_notation(atom_key):
      
      split = list(atom_key)
      element = ''
      number = False
      count = 0
      while number is False:
          element = "".join((element, split[count]))
          count += 1
          if is_number(split[count]) is True:
              number = True
      # In case of for example Material Studio output, integers can also be
      # in the beginning of the string. As the dlf_notation decipher function
      # is very general in use, we have to make sure these integers are deleted.
      # In standard DL_F notation the string will never start with integer so it
      # will not affect the functionality towards it.
      # EDIT2: also the '?' atoms, you can delete them manually or somewhere else
      element = "".join(i for i in element if not is_number(i))
      element = "".join(i for i in element if i != '?')
      return element
docs: Return element for atom key using DL_F notation.