The worked path in

Build on NULLIUS

Everything reduces to cells, turns and receipts — so building an app is defining a cell's predicate, issuing capabilities, and signing turns. Here is the whole loop in real code you can paste into node.

1 · Make a cell with a predicate

A budget that cannot be exceeded is just a cell whose program says so.

import { genesis, makeCell, faucet } from "./kernel/executor.js";
import { genKeypair, randomHex } from "./kernel/crypto.js";

const owner  = await genKeypair();
const secret = await randomHex(32);      // the cell's cap-root secret

let state = genesis();
state.cells["cell:treasury"] = makeCell("cell:treasury", {
  owner: owner.pub, rootSecret: secret,
  predicate: { kind: "budget", asset: "GRAIN", limit: 500 }, // lifetime cap
});
state = (await faucet(state, { recipient: "cell:treasury", amount: 1000 })).state;

2 · Issue a capability, then attenuate it

Narrow, never amplify. Hand an agent a capped token and it physically cannot exceed it.

import { issue, attenuate } from "./kernel/caps.js";

const full = await issue(secret, {
  cell: "cell:treasury", rights: ["transfer", "write"], holder: owner.pub,
});
// give an agent transfer-only, max 50 per turn
const agentCap = await attenuate(full, {
  rights: ["transfer"],
  caveat: { type: "max_amount", limit: 50 },
});

3 · Sign a turn and apply it

import { signTurn, applyTurn } from "./kernel/executor.js";

const turn = await signTurn({
  actorPub: owner.pub, actorPriv: owner.priv, nonce: await randomHex(),
  actions: [{ verb: "transfer", asset: "GRAIN", amount: 40,
              from: "cell:treasury", to: "cell:agent", cap: agentCap }],
});
const r = await applyTurn(state, turn);
if (r.ok) { state = r.state; console.log("receipt", r.receipt.q); }
else      { console.log("refused:", r.reason); } // the rejection is the product

That is the entire builder surface: define a predicate, issue and attenuate capabilities, sign turns. The playground is these three steps wired to buttons. The organs (credit lines, encrypted groups, custody handoffs) are each a few lines on this same executor — no new kernel entry.

The protocol, in seven rungs → Glossary