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#diagram(debug: 0,
  node((0,0), name: <A>)[$A$],
  node((1,0), name: <B>)[$B$],
  node((2,0), name: <C>)[$C$],
  node((3,0), name: <D>)[$D$],
  node((4,0), name: <E>)[$E$],
  node((5,0), name: <F>)[$F$],
  edge(<A>, "->", <B>, bend:  30deg),
  edge(<A>, "<-", <B>, bend: -30deg),
  edge(<B>, "->", <C>, bend:   0deg),
  edge(<E>, "->", <F>, bend:  30deg),
  edge(<E>, "<-", <F>, bend: -30deg),
  edge(<D>, "->", <E>, bend:   0deg),
  node(name: <G>, stroke: 1pt + black, enclose: (<A>, <B>, <C>)),
)