def _iso_inv_map(d):
    """
    Augment *d* with inverse mappings.

    Assumes *d* is not a multigraph.
    """
    _d = {}
    for src, d2 in d.items():
        for tgt, data in d2.items():
            if tgt is not None and src != tgt:
                if tgt not in _d:
                    _d[tgt] = {}
                _d[tgt][src] = '--' + data
    for k, d2 in _d.items():
        d[k].update(d2)
