Typst - Only number labeled equations

only-number-labeled.typ

// MIT No Attribution
// 
// Copyright (c) 2023 Pg Biel
// 
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#let labeled-counter = counter("labeled-eq")
#let should-number = state("should-number-eq", (:))
#show math.equation: it => {
  if it.has("label") {
    let eq-number = counter(math.equation).get().first()
    let number = labeled-counter.get().first()

    should-number.update(numbered => {
      // Associate this equation number with a new labeled-only number
      numbered.insert(str(eq-number), number + 1)
      numbered
    })
    labeled-counter.step()
  }
  it
}

#set math.equation(numbering: n => {
  let num = should-number.final().at(str(n), default: none)
  if num != none {
    numbering("(1)", num)
  }
})

$ a $ <a>

$ b $

$ c $ <c>

$ d $

$ e $ <e>

Hello tfhe @a

Hello the @c

the @e