docs: def _generate_alphabet_dict(iterable, reserved_tokens=None):
    """Create set of characters that appear in any element in the iterable."""
    if reserved_tokens is None:
      reserved_tokens = RESERVED_TOKENS
    alphabet = {c for token in iterable for c in token}
    alphabet |= {c for token in reserved_tokens for c in token}
    alphabet |= _ESCAPE_CHARS  # Add escape characters to alphabet set.
    return alphabet