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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 overlay_depth(obj):
 """ Computes the depth of a DynamicMap overlay if it can be determined otherwise return None. """

    if isinstance(obj, DynamicMap):
        if isinstance(obj.last, CompositeOverlay):
            return len(obj.last)
        elif obj.last is None:
            return None
        return 1
    else:
        return 1