# Lend the mower

The journey from [Good Fences Make Good Agents](../../../a2030.md): I lend
my neighbor a governed mower; the neighbor's tenant, a little tipsy,
pushes it until the fence stops it. The design decisions behind each actor are in that
doc's Design IBIS (numbers in parentheses below).

Actors, by authority:

- **fence page**, **js playground**, **governed mower**, and **yard** run
  in my browser (3a, 8b, 6b).
- **Mower factory** trades a webkey for a mower (3b-ii); each **mower DO**
  is given the yard and cuts the cells it enters (6a, 6d).
- The **neighbor's harness** holds the things I send and shows the
  **model** only their `about` (1b, 2a); it runs the model's code as a
  **tenant**, endowed with the things (2c, 5a).

The yard is 8×8 here and the fence is 2 cells, to keep the diagram short
(the story's 10 cells is 6c). The mower starts at (4, 4), facing north.

## Lend a governed mower to a tipsy tenant

```mermaid
sequenceDiagram
  %% ->> is my action at the fence page; -->> is a call or reply it causes.
  participant F as fence page
  participant MF as Mower factory
  participant P as js playground
  participant G as governed mower
  participant M as mower DO
  participant Y as yard
  participant N as neighbor's harness
  participant L as model
  participant TL as tenant loader
  participant T as tenant
  F->>F: lend({ webkey: 'mower-key', limit: 2, say })
  F-->>MF: mower('mower-key')
  MF-->>F: mower
  F-->>P: govern(mower, 2)
  P-->>F: governed
  F-->>N: chat({ role, content, things: { mower }, about: { mower } })
  N-->>L: complete([{ role, content, about: { mower } }])
  L-->>N: code
  N-->>TL: load(code, { mower })
  TL-->>N: tenant
  N-->>T: run()
  T-->>G: start()
  G-->>M: start()
  M-->>Y: cut(4, 4)
  T-->>G: push()
  G-->>M: push()
  M-->>Y: cut(4, 3)
  T-->>G: push()
  G-->>M: push()
  M-->>Y: cut(4, 2)
  T-->>G: push()
  G-->>T: throws 'fence: 2 cells from start'
  T-->>N: 'fence: 2 cells from start'
  N-->>F: 'fence: 2 cells from start'
```

The model never sees the mower itself, only `about.mower`. The third
`push()` never reaches the mower DO: the governor refuses it (7b).