Hashing tuples of tuples is seemingly very slow

General Usage Performance
2

Not an answer to your question, but this usage of tuples looks overly complicated. Why not just

 if (p1, p2) in seen
    return score(p1),1
end        
push!(seen, (p1, p2))

It is possible, that Tuple([Tuple(p1), Tuple(p2)]) construction creates intermediate arrays.

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