ocaml graph.cma tests/check.ml
iter:
pre 3
pre 5
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pre 2
post 2
post 4
post 5
pre 6
post 6
post 3
prefix:
pre 3
pre 6
pre 5
pre 4
pre 2
step:
visit 6
visit 5
visit 4
visit 2
visit 3
visit 1