mother |sally> => |trude>
father |sally> => |tom>

mother |erica> => |trude>
father |erica> => |tom>

mother |trude> => |sara>
father |trude> => |sam>

mother |tom> => |ruth>
father |tom> => |mike>

mother |ruth> => |gina>

mother |mike> => |mary>
father |mike> => |mark>