Create an incremental unifier

[ngeiswei]

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Unification is inherently costly (NP in the worst case, I think, don't quote me on that). However queries are often similar, or their complexity incrementally builds up due to how the URE evolves inference trees.

So this would be a generalization of memoizing unification, see issue https://github.com/opencog/atomspace/issues/1913 but is much more advanced, it's pretty much a research project, might already have some literature on the subject, TBD.

[ngeiswei]

Looking into https://en.wikipedia.org/wiki/Incremental_computing could provide some help.