Grammar

One line is one statement. A call at runtime is =>. An import is rule a -> b. A file that still says a -> b is read as a call.

key: Type "Title" "Technology" "Description"	A box. The key is how every other line names it.
a => b "label" "HTTPS"	A call at runtime. The caller waits. a -> b still reads as a call.
a --> b "label"	A message. Nobody waits.
rule a -> b	An import, and only on a code board. The word rule is what makes -> an import.
rule a -/-> b	An import that is not allowed.
flow "Name": a => b "label", b => c	A path. Cite the label so two arrows between the same boxes stay two steps.
inside db boards/api/db.hunch	The board inside that box. One level is boards/api.hunch. The next is boards/api/db.hunch.

A diagram inside a diagram

On the outer board, the box has a key, like api.
One line on that file points at the inner file: inside api boards/api.hunch. The word before the path is the key.
That file is a whole board. It can be another diagram, the code inside the box, or its screens.
A box there goes one level down: inside db boards/api/db.hunch. Two levels is enough.
On the board, the box says Open inside. The first time, an empty file asks: Diagram, Code architecture, or UI sketch.

System

title Shop
web: Web app "Storefront" "Next.js" "The shop window"
api: Service "Orders" "Go" "Takes the order"
db: Relational DB "Orders DB" "Postgres"
web => api "Places order" "HTTPS"
api => db "Saves"
flow "Checkout": web => api "Places order", api => db "Saves"
inside api boards/api.hunch

Code inside a box

title Orders
style clean
http: Route handler "POST /orders" [layer=adapters path=src/http]
order: Use case "Place order" [layer=use-cases path=src/order]
http => order "calls"
rule adapters -> use-cases
rule use-cases -/-> adapters
inside order boards/api/order.hunch

Screens

title Shop screens
mode ui
cart-total: Stat "Total $68"
pay: Button "Pay" [primary calls="POST /orders"]
frame "Cart" [device=mobile route="/cart" empty="Your cart is empty"]: cart-total, pay

The long guide is the skill. The folder still works without the inside line: boards/<key>.hunch, then boards/<key>/<box>.hunch.