gument:
        mode -- one of the strings 'standard', 'logo' or 'world'

        Mode 'standard' is compatible with turtle.py.
        Mode 'logo' is compatible with most Logo-Turtle-Graphics.
        Mode 'world' uses userdefined 'worldcoordinates'. *Attention*: in
        this mode angles appear distorted if x/y unit-ratio doesn't equal 1.
        If mode is not given, return the current mode.

             Mode      Initial turtle heading     positive angles
         ------------|-------------------------|-------------------
          'standard'    to the right (east)       counterclockwise
            'logo'        upward    (north)         clockwise

        Examples:
        >>> mode('logo')   # resets turtle heading to north
        >>> mode()
        'logo'
        NŠ