docs: def check_allowed_letters(seq, allowed_letters_as_set):
      """Validate sequence: Rise an error if sequence contains undesirable letters.  """
  
      # set of unique letters in sequence
      seq_set = set(seq)
  
      not_allowed_letters_in_seq = [x for x in seq_set if str(x).upper() not in allowed_letters_as_set]
  
      if len(not_allowed_letters_in_seq) > 0:
          raise forms.ValidationError(
              "This sequence type cannot contain letters: " + ", ".join(not_allowed_letters_in_seq))