def remove_chars(string: str) -> str:
    """Remove all characters but strings.

    Args:
        string: <str> input string sequence.

    Returns:
        string: <str> sorted only letters string.

    Examples:
        >>> assert remove_chars('.tree1') == 'tree'
    """

    return ''.join(
        filter(lambda letter: letter.isalpha() or letter.isspace(), string)
    )