<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def join(chord_root, quality='', extensions=None, bass=''):
 r"""Join the parts of a chord into a complete chord label. Parameters chord_root : str Root pitch class of the chord, e.g. 'C', 'Eb' quality : str Quality of the chord, e.g. 'maj', 'hdim7' (Default value = '') extensions : list Any added or absent scaled degrees for this chord, e.g. ['4', '\*3'] (Default value = None) bass : str Scale degree of the bass note, e.g. '5'. (Default value = '') Returns ------- chord_label : str A complete chord label. """

    chord_label = chord_root
    if quality or extensions:
        chord_label += ":%s" % quality
    if extensions:
        chord_label += "(%s)" % ",".join(extensions)
    if bass and bass != '1':
        chord_label += "/%s" % bass
    validate_chord_label(chord_label)
    return chord_label