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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 default_unit_label(axis, unit):
 """Set default label for an axis from a `~astropy.units.Unit` If the axis already has a label, this function does nothing. Parameters axis : `~matplotlib.axis.Axis` the axis to manipulate unit : `~astropy.units.Unit` the unit to use for the label Returns ------- text : `str`, `None` the text for the new label, if set, otherwise `None` """

    if not axis.isDefault_label:
        return
    label = axis.set_label_text(unit.to_string('latex_inline_dimensional'))
    axis.isDefault_label = True
    return label.get_text()