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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _validate_series(series, label, add_error):
 """Check that the given series is valid. Use the given label (e.g. "machine X" or just "bundle") to describe possible errors. Use the given add_error callable to register validation error. """

    if series is None:
        return
    if not isstring(series):
        add_error('{} series must be a string, found {}'.format(label, series))
        return
    if series == 'bundle':
        add_error('{} series must specify a charm series'.format(label))
        return
    if not references.valid_series(series):
        add_error('{} has invalid series {}'.format(label, series))