context、计数器与状态：Typst 如何“多遍”编译

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#set page(width: 14cm, height: auto, margin: 1cm)
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// Style values like `text.size` exist only inside `context`.
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#set text(size: 14pt)
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#let size-now = context text.size
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// The result is content that is resolved when it is placed.
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#assert.eq(type(size-now), content)
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#text(size: 20pt)[在 20pt 里：#size-now]
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// `here()` is the current location; `locate`-style queries read its page.
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#context [第 #here().page() 页，位置 #here().position()。]
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// `context` also gives access to `measure` and `layout`.
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#context {
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  let w = measure([中文abc]).width
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  [这行文字宽 #calc.round(w.pt(), digits: 1) pt。]
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}
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#pagebreak()
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#context [这是第 #here().page() 页。]

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warning: document did not converge within five attempts
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 = hint: see 2 additional warnings for more details
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 = hint: see https://typst.app/help/convergence for help
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warning: value of `state("s")` did not converge
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  ┌─ lab/ch07/diverge.warn.typ:4:18
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  │
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4 │ #context s.update(s.final() + 1)
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  │                   ^^^^^^^^^
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  │
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  = hint: the following values were observed:
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    - run 1: `0`
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    - run 2: `1`
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    - run 3: `2`
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    - run 4: `3`
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    - run 5: `4`
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    - final: `5`
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  = hint: see https://typst.app/help/state-convergence for help
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warning: value of `state("s")` did not converge
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  ┌─ lab/ch07/diverge.warn.typ:5:9
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  │
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5 │ #context s.get()
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  │          ^^^^^^^
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  │
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  = hint: the following values were observed:
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    - run 1: `0`
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    - run 2: `1`
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    - run 3: `2`
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    - run 4: `3`
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    - run 5: `4`
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    - final: `5`
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  = hint: see https://typst.app/help/state-convergence for help

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#set page(width: 12cm, height: auto, margin: 8pt, numbering: "1")
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#set heading(numbering: "1.1")
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#let c = counter("c")
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= 甲
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== 子
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== 子二
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= 乙
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== 子三
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#context [
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  heading：#counter(heading).get() \
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  display()：#counter(heading).display() \
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  display("1.1")：#counter(heading).display("1.1")
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]
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#c.step() #c.step() #c.step()
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#context [get：#c.get() \ display("I")：#c.display("I")]
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#c.update((2, 5))
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#context [update((2, 5))：#c.get() \ display("1.1")：#c.display("1.1")]
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#c.step(level: 2)
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#context [step(level: 2)：#c.get()]
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#c.step(level: 1)
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#context [step(level: 1)：#c.get()]
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#counter(page).update(10)
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#pagebreak()
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#context [
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  页码计数器：#counter(page).get() \
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  物理页：#here().page() \
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  display("i")：#counter(page).display("i")
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]

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1甲
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1.1 子
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1.2 子二
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2乙
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2.1 子三
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heading：(2, 1)
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display()：2.1
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display(“1.1”)：2.1
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get：(3,)
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display(“I”)：III
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update((2, 5))：(2, 5)
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display(“1.1”)：2.5
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step(level: 2)：(2, 6)
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step(level: 1)：(3,)
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页码计数器：(11,)
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物理页：2
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display(“i”)：xi

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#set page(width: 12cm, height: auto, margin: 8pt)
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#figure(rect(width: 1cm, height: 1cm), caption: [甲])
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#figure(
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  table(
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    columns: 1,
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    [x],
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  ),
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  caption: [乙],
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)
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#figure(rect(width: 1cm, height: 1cm), caption: [丙])
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#context [
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  图 #counter(figure.where(kind: image)).final().first()，
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  表 #counter(figure.where(kind: table)).final().first()，
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  全部 figure #counter(figure).final()
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]

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#set page(width: 12cm, height: auto, margin: 8pt)
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#let s = state("s", 0)
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#let x = s.update(5)
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未放置：#context s.get() \
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#x
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放置之后：#context s.get() \
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#context {
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  s.update(9)
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  [同一个 context 里 update 之后立刻读：#s.get()]
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} \
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下一个 context：#context s.get()
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#let log = state("log", ())
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#let note(t) = log.update(l => l + (t,))
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#note("甲") #note("乙")
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#context [此刻：#log.get().join("、")]
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#note("丙")
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#context [
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  此刻：#log.get().join("、") \
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  最终：#log.final().join("、")
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]

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#set page(
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  width: 12cm,
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  height: 7cm,
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  margin: (top: 2cm, rest: 1cm),
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  header: context {
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    // The header sees the document as of its own page.
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    let here-page = here().page()
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    let starts = query(heading.where(level: 1)).any(h => (
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      h.location().page() == here-page
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    ))
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    if not starts {
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      let before = query(heading.where(level: 1).before(here()))
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      if before.len() > 0 { emph(before.last().body) }
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    }
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  },
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)
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#set heading(numbering: "1.")
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#show heading.where(level: 1): it => pagebreak(weak: true) + it
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= 第一章
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#lorem(80)
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= 第二章
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#lorem(120)

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/// Running header: current chapter on the left, document title on the right.
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/// Suppressed on pages that start a level-1 heading.
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#let xuan-header() = context {
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  let starts = query(heading.where(level: 1)).any(h => (
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    h.location().page() == here().page()
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  ))
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  if not starts {
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    let before = query(heading.where(level: 1).before(here()))
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    let chapter = if before.len() > 0 { before.last().body } else { none }
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    let title = document.title
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    set text(size: 0.85em)
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    grid(
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      columns: (1fr, 1fr),
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      align(left, chapter), align(right, title),
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    )
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    v(-0.5em)
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    line(length: 100%, stroke: 0.4pt)
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  }
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}
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/// Footer: `第 x 页 / 共 y 页` (zh) or `x / y` (en).
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#let xuan-footer(lang: "zh") = context {
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  let x = counter(page).get().first()
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  let y = counter(page).final().first()
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  set text(size: 0.85em)
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  align(center, if lang == "zh" [第 #x 页 / 共 #y 页] else [#x / #y])
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}

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// Numbered environments (theorem, lemma, ...) that share one counter,
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// restart at every level-1 heading and can be referenced with `@label`.
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#let theorem-kind = "xuan-theorem"
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/// "<chapter>.<n>" for the environment figure `el`.
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#let env-number(el) = {
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  let loc = el.location()
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  numbering(
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    "1.1",
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    counter(heading).at(loc).first(),
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    el.counter.at(loc).first(),
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  )
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}
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/// Build an environment function `(title: none, body)`.
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#let make-env(supplement-zh, supplement-en) = {
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  (title: none, body) => figure(
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    kind: theorem-kind,
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    supplement: context {
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      if text.lang == "zh" { supplement-zh } else { supplement-en }
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    },
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    caption: title,
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    outlined: false,
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    gap: 0pt,
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    body,
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  )
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}
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#let theorem = make-env("定理", "Theorem")
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#let lemma = make-env("引理", "Lemma")
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#let corollary = make-env("推论", "Corollary")
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#let definition = make-env("定义", "Definition")
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#let example = make-env("例", "Example")
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/// Unnumbered proof ending with a right-aligned ∎.
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#let proof(body) = {
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  block(width: 100%, above: 0.8em, below: 0.8em, {
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    set par(first-line-indent: 0pt)
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    emph(context if text.lang == "zh" [证明.] else [Proof.])
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    h(0.5em)
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    body
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    h(1fr)
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    sym.qed
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  })
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}
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/// Show rules for the environments; applied by `xuan-base`.
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#let theorem-rules(body) = {
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  show heading.where(level: 1): it => {
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    counter(figure.where(kind: theorem-kind)).update(0)
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    it
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  }
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  show figure.where(kind: theorem-kind): it => block(
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    width: 100%,
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    above: 0.8em,
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    below: 0.8em,
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    {
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      set align(left)
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      set par(first-line-indent: 0pt)
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      strong[#it.supplement #env-number(it)]
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      if it.caption != none [ (#it.caption.body)]
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      [.]
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      h(0.5em)
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      emph(it.body)
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    },
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  )
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  show ref: it => {
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    let el = it.element
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    if el != none and el.func() == figure and el.kind == theorem-kind {
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      let supp = if it.supplement == auto { el.supplement } else {
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        it.supplement
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      }
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      link(el.location(), [#supp~#env-number(el)])
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    } else { it }
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  }
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  body
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}

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#import "@local/xuan:0.1.0": *
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#show: xuan-base
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= 第一章
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#theorem[甲] <t1>
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#lemma[乙] <t2>
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#proof[略。]
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#example[丙] <t3>
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= 第二章
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#theorem[丁] <t4>
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#context {
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  // One counter is shared by all environments and restarts at each chapter.
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  let n(l) = counter(figure.where(kind: "xuan-theorem")).at(l).first()
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  assert.eq((n(<t1>), n(<t2>), n(<t3>), n(<t4>)), (1, 2, 3, 1))
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  // `proof` is unnumbered.
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  assert.eq(query(figure.where(kind: "xuan-theorem")).len(), 4)
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}