Outline

To enable outline support, you may either use outline: auto on an element that already supports references - in which case it will simply reuse the reference supplement and numbering in the outline - or use outline: (caption: fields => content), which will show an extra caption beside supplement and numbering if they exist, otherwise (if the element doesn't support references, or uses (custom: ...) for references) it will simply display the caption by itself.

Note that the user doesn't need #show: e.prepare() for outline support to work, but it's good practice since it's needed for references.

The user may then display the element's outline using #outline(target: e.selector(elem, outline: true)).

#import "@local/elembic:0.0.1" as e: field

#show: e.prepare()

#let theorem = e.element.declare(
  "theorem",
  prefix: "my-package",

  display: it => [*Theorem #e.counter(it).display("1"):* #text(fill: it.fill)[#it.body]],

  reference: (
    supplement: [Theorem],
    numbering: "1"
  ),

  outline: auto,

  fields: (
    e.field("body", content, required: true),
    e.field("fill", e.types.paint, doc: "The text fill.", default: red),
  )
)

#outline(target: e.selector(theorem, outline: true))

#theorem(label: <my-thm>)[*Hello world*]
#theorem(fill: blue, label: <other-thm>)[*1 + 1 = 2*]