def build_vf_node(vf):
    """
    Convert a VulnerabilityFunction object into a Node suitable
    for XML conversion.
    """
    nodes = [Node('imls', {'imt': vf.imt}, vf.imls),
             Node('meanLRs', {}, vf.mean_loss_ratios),
             Node('covLRs', {}, vf.covs)]
    return Node(
        'vulnerabilityFunction',
        {'id': vf.id, 'dist': vf.distribution_name}, nodes=nodes)
Convert a VulnerabilityFunction object into a Node suitable
    for XML conversion.