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