Retrieve documents from the knowledge graph and expand the graph around them.

By default only the requested document(s) are returned. Each --expand-* flag
follows one edge kind out from every seed and pulls the reached documents in:

1. Seed documents (-k, search, or stdin)
2. Children (--expand-includes N levels down)
3. Parent context (--expand-included-by N levels up)
4. Outbound reference links (--expand-references N levels)
5. Inbound reference links (--expand-referenced-by N levels)

Without -k, --filter, --lexical/--fuzzy, or stdin, the command exits with an
error. -k accepts repeats and resolves to $key (1 key = $eq, 2+ = $in). Use
--filter / --included-by / --references / etc. to narrow the seed set; explicit
keys are intersected with the filter. Empty intersection produces an empty result.
With --lexical/--fuzzy the seeds are found by relevance within that set; --limit
caps the seeds before expansion.
