.Str: ClassType
.Str.
    '''
    Return a capitalized version of the string.

    More specifically, make the first character have upper case and the rest lower
    case.
    '''
    '''erg
    assert "hello".capitalize() == "Hello"
    assert "HELLO".capitalize() == "Hello"
    '''
    capitalize: (self: .Str) -> .Str
    '''
    Return a version of the string suitable for caseless comparisons.
    '''
    '''erg
    assert "camelCase".casefold() == "camelcase"
    assert "CamelCase".casefold() == "camelcase"
    assert "FULLCAPS".casefold() == "fullcaps"
    assert "snake_case".casefold() == "snake_case"
    '''
    casefold: (self: .Str) -> .Str
    '''
    Return a centered string of length width.

    Padding is done using the specified fill character (default is a space).
    '''
    '''erg
    assert "hello".center(10) == "  hello   "
    assert "hello".center(10, "-") == "--hello---"
    '''
    center: (self: .Str, width: Int, fillchar := .Str) -> .Str