Handling cyclic query-graphs?

[colleenXu]

We may want Retriever to be able to handle these? And we discussed in Slack making an issue to remember this.

BTE doesn't handle cyclic query-graphs. I found these old issues where "we don't want to handle cyclic query-graphs" was discussed. I found some relevant discussion when implementing is_set support.

• comment on result-assembly for multi-hop queries
  • made a query-handler algorithm that was implemented: discussed here and here
• using tree-traversal to assemble results
  • related to is_set code
• discussing cyclic query-graphs and whether to handle them or not