Merges graphs to remove redundant nodes. This takes graphs and merges them, pruning redundant
nodes within the graphs and between graphs previously merged. It remembers graphs it has
previously seen to allow nodes to be reused across multiple graphs.
Merge and simplify a graph. This will coalesce redundant nodes (equivalent labels and
outgoing edge destinations), and will prefer to use nodes from graphs seen previously.
This allows deduplication across multiple graphs.
Noteo: edge labels are ignored for the purpose of merging.