def METHOD_NAME():
  """
  Test is in
  iotbx/regression/tst_all_chain_ids.py
  There should not be leading whitespace for one letter chain ids.
  Returns all possible 2 character chain IDs for PDB format files.
  In general, returns single character chains first.
  Also tries to use all combinations of uppercase/numbers before going to lower case.
  """
  chars = string.ascii_uppercase+string.digits
  lowerchars = string.ascii_lowercase
  both_char_upper = __permutations(iterable = chars, r = 2)
  second_char_lower = product(chars, lowerchars)
  first_char_lower = product(lowerchars, chars)
  both_char_lower = __permutations(iterable = lowerchars, r = 2)
  # result = [" "+c for c in chars]+[" "+c for c in lowerchars]+\
  result = [c for c in chars]+[c for c in lowerchars]+\
         ["".join(p) for p in both_char_upper]+\
         ["".join(p) for p in second_char_lower]+\
         ["".join(p) for p in first_char_lower]+\
         ["".join(p) for p in both_char_lower]
  return result