```
data(learning.test)
res = gs(learning.test, optimized = TRUE)
ntests(res)
res = set.arc(res, "A", "B")
ord = node.ordering(res)
ord
## partial node ordering saves us two tests in the v-structure
## detection step of the algorithm.
ntests(gs(learning.test, blacklist = ordering2blacklist(ord)))
tiers2blacklist(list(LETTERS[1:3], LETTERS[4:6]))
```

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