def METHOD_NAME():
  nff_files = []
  for file in os.listdir(reference_tables_directory):
    fn = file.lower().capitalize()
    if (fn[-4:] == ".nff"): nff_files.append(file)
  tables = [0] * 120
  for file in nff_files:
    f = join_open(reference_tables_directory, file, "r")
    header = f.readline()
    table = f.readlines()
    f.close()
    Symbol = header[1:3].strip()
    Z = int(header[7:9])
    assert len(Symbol) > 0
    assert Symbol[0] in 'abcdefghijklmnopqrstuvwxyz'
    assert Symbol[-1] in 'abcdefghijklmnopqrstuvwxyz'
    assert Z > 0 and Z < len(tables)
    assert tables[Z] == 0
    Symbol = Symbol.capitalize()
    tables[Z] = (Symbol, table)
  Z = tables[1:].index(0) + 1
  rest = tables[Z:]
  assert rest == [0] * len(rest)
  tables = tables[:Z]
  return tables