<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def raise_for_old_graph(graph):
 """Raise an ImportVersionWarning if the BEL graph was produced by a legacy version of PyBEL. :raises ImportVersionWarning: If the BEL graph was produced by a legacy version of PyBEL """

    graph_version = tokenize_version(graph.pybel_version)
    if graph_version < PYBEL_MINIMUM_IMPORT_VERSION:
        raise ImportVersionWarning(graph_version, PYBEL_MINIMUM_IMPORT_VERSION)