How to fix misbehaving arc in fletcher?

Questions
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1

I currently have the following diagram

feyn1
feyn1511×628 92.7 KB

from the code:

#set page(
  width: auto,
  height: auto,
  margin: 2mm,
)

#import "@preview/fletcher:0.5.8" as fletcher: diagram, node, edge, cetz
#import fletcher.shapes: circle, pill, diamond

#diagram(
  debug: 0,
  node-shape: circle,
  node-inset: 1.5mm,
  node-outset: 0mm,
  node-fill: none,
  axes: (ltr, btt),
  let diam-radius = 0.6,
  let v2-pos = (3, 2),
  let v3-pos = (3, -2),
  node((-diam-radius, 0), $a$, stroke: black, name: <a>),
  node((diam-radius, 0), $c$, stroke: black, name: <c>),
  node((0, diam-radius), $b$, stroke: black, name: <b>),
  node((0, -diam-radius), $d$, stroke: black, name: <d>),
  //
  node((v2-pos.at(0) - diam-radius, v2-pos.at(1)), $e$, stroke: black, name: <e>),
  node((v2-pos.at(0) + diam-radius, v2-pos.at(1)), $g$, stroke: black, name: <g>),
  node((v2-pos.at(0), v2-pos.at(1) + diam-radius), $f$, stroke: black, name: <f>),
  node((v2-pos.at(0), v2-pos.at(1) - diam-radius), $h$, stroke: black, name: <h>),
  //
  node((v3-pos.at(0) - diam-radius, v3-pos.at(1)), $i$, stroke: black, name: <i>),
  node((v3-pos.at(0) + diam-radius, v3-pos.at(1)), $k$, stroke: black, name: <k>),
  node((v3-pos.at(0), v3-pos.at(1) + diam-radius), $j$, stroke: black, name: <j>),
  node((v3-pos.at(0), v3-pos.at(1) - diam-radius), $l$, stroke: black, name: <l>),
  // 4-element sets partition
  node([], enclose: (<a>, <b>, <c>, <d>), stroke: orange, shape: diamond),
  node([], enclose: (<e>, <f>, <g>, <h>), stroke: orange, shape: diamond),
  node([], enclose: (<i>, <j>, <k>, <l>), stroke: orange, shape: diamond),
  // Easy 2-element sets
  node([], enclose: (<a>, <c>), stroke: blue, shape: pill),
  node([], enclose: (<h>, <j>), stroke: blue, shape: pill),
  node([], enclose: (<e>, <i>), stroke: blue, shape: pill),
  node([], enclose: (<g>, <k>), stroke: blue, shape: pill),
  // 2-element set for b, f nodes
)

Now, I need to create two more blue contours, to circle around b, f and d, l respectively, preferably without overlapping the other blue contours. I assume the node system isn’t going to work, so I am now trying to get edges to work. The solution I am trying to attempt is stitch together a bunch of arcs. The idea I have for say the b and l nodes is:

  1. for the b node, place a semi-circular arc below the node, using the bend parameter.
  2. Place two vertical lines on top of the semicircle.
  3. Place two arcs on the vertical lines to have them bend right.
  4. Place two horizontal lines after the arcs.
  5. Lastly, place a right semicircular arc on the f node.

However, when I try and actually start with the semicircular arc under the b node, I get:

feyn2
feyn2511×628 93.1 KB

with the code being:

#diagram(
  debug: 0,
  node-shape: circle,
  node-inset: 1.5mm,
  node-outset: 0mm,
  node-fill: none,
  axes: (ltr, btt),
  let diam-radius = 0.6,
  let v2-pos = (3, 2),
  let v3-pos = (3, -2),
  node((-diam-radius, 0), $a$, stroke: black, name: <a>),
  node((diam-radius, 0), $c$, stroke: black, name: <c>),
  node((0, diam-radius), $b$, stroke: black, name: <b>),
  node((0, -diam-radius), $d$, stroke: black, name: <d>),
  //
  node((v2-pos.at(0) - diam-radius, v2-pos.at(1)), $e$, stroke: black, name: <e>),
  node((v2-pos.at(0) + diam-radius, v2-pos.at(1)), $g$, stroke: black, name: <g>),
  node((v2-pos.at(0), v2-pos.at(1) + diam-radius), $f$, stroke: black, name: <f>),
  node((v2-pos.at(0), v2-pos.at(1) - diam-radius), $h$, stroke: black, name: <h>),
  //
  node((v3-pos.at(0) - diam-radius, v3-pos.at(1)), $i$, stroke: black, name: <i>),
  node((v3-pos.at(0) + diam-radius, v3-pos.at(1)), $k$, stroke: black, name: <k>),
  node((v3-pos.at(0), v3-pos.at(1) + diam-radius), $j$, stroke: black, name: <j>),
  node((v3-pos.at(0), v3-pos.at(1) - diam-radius), $l$, stroke: black, name: <l>),
  // 4-element sets partition
  node([], enclose: (<a>, <b>, <c>, <d>), stroke: orange, shape: diamond),
  node([], enclose: (<e>, <f>, <g>, <h>), stroke: orange, shape: diamond),
  node([], enclose: (<i>, <j>, <k>, <l>), stroke: orange, shape: diamond),
  // Easy 2-element sets
  node([], enclose: (<a>, <c>), stroke: blue, shape: pill),
  node([], enclose: (<h>, <j>), stroke: blue, shape: pill),
  node([], enclose: (<e>, <i>), stroke: blue, shape: pill),
  node([], enclose: (<g>, <k>), stroke: blue, shape: pill),
  // 2-element set for b, f nodes
  edge((-0.35, diam-radius), (0.35, diam-radius), stroke: red + 0.35mm, bend: 0deg),
  edge((-0.35, diam-radius), (0.35, diam-radius), stroke: green + 0.35mm, bend: 10deg),
}

where the red line is the starting point and my intent was to bend it until it curved under the b node. For some reason, its positioning changes entirely, as seen with the green arc.

Can anyone help me figure out what is going on here? Thank you.

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