npx hstp;

As steps:

0)  - Baby wants to drink milk.
1)  - Baby cries to the mom.
3)  - Mom see the baby is crying.
4)     - Mom checks the fridge. Mom sees the milk is empty. (Mother is only trusting the Father)
5)        - Mom calls the father.
6)        - Father calls the milkman.
7)        - Milkman delivers the milk to father.
8)        - Father delivers the milk to mom.
9)  - Mom gives the milk to the baby.
10) - Baby drinks the milk.
11) - Baby is happy.
12) - Baby sleeps.
13) - Mother see the baby is happy and sleeps.
14) - In order to be able to contact the milkman again, the mother asks the father to tell her that she wants the milkman to save the address of the house and the mobile phone of the mother.
15)     - Mother calls the father.
16)     - Father calls the milkman.
17)     - Milkman saves the address of the house and the mobile phone of the mother.

Oops, tomorrow baby wakes up and cries again,
0) - Baby wants to drink milk.
1) - Baby cries to the mom.
2) - Mom see the baby is crying.
3)    - Mom checks the fridge. Mom sees the milk is empty. (Mother is trusting the Father had right decision in the first place by giving the address to the milkman, and the milkman had right decision in the first place by saving the address of the house and the mobile phone of the mother.)
4)    - Mother calls the milkman (Mother is trusting the Father's decision only)
5)    - Milkman delivers the milk to mom.
6) - Mom gives the milk to the baby.
7) - Baby drinks the milk.
8) - Baby is happy.
9) - Baby sleeps.
10) - Mother see the baby is happy and sleeps.
11) - Mother is happy and the mother trust the milkman now.

// SPDX-License-Identifier: GNU-3.0-or-later
pragma solidity ^0.8.0;

import "./HSTP.sol";
import "./ERC165.sol";

enum Operation {
    Query,
    Mutation
}

struct Response {
    uint256 status;
    string body;
}

struct Registry {
    HSTP resolver;
}

// HSTP/Router.sol
abstract contract Router is ERC165 {
    event Log(address indexed sender, Operation operation, bytes payload);
    event Register(address indexed sender, Registry registry);
    mapping(string => Registry) public routes;

    function reply(string memory name, Operation _operation, bytes memory payload) public virtual payable returns(Response memory response) {
        emit Log(msg.sender, _operation, payload);
        // Traverse upwards and downwards of the tree.
        // Tries to find the closest path for given operation.
        // If the route is registered on HSTP node, reply from children node.
        // If the node do not have the route on this node, ask for parent.
        if (routes[name]) {
            if (_operation == Operation.Query) {
                return this.query(payload);
            } else if (_operation == Operation.Mutation) {
                return this.mutation(payload);
            }
        }
        return super.reply(name, _operation, payload);
    }

    function query(string memory name, bytes memory payload) public view returns (Response memory) {
        return routes[name].resolver.query(payload);
    }

    function mutation(string memory name, bytes memory payload) public payable returns (Response memory) {
        return routes[name].resolver.mutation(payload);
    }

    function register(string memory name, HSTP node) public {
        Registry memory registry = Registry({
            resolver: node
        });
        emit Register(msg.sender, registry);
        routes[name] = registry;
    }

    function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
        return interfaceId == type(HSTP).interfaceId;
    }
}

// SPDX-License-Identifier: GNU-3.0-or-later
pragma solidity ^0.8.0;

import "./Router.sol";

// Stateless Hyper Service Transfer Protocol for on-chain services.
// Will implement: EIP-4337 when it's on final stage.
// https://github.com/ethereum/EIPs/blob/master/EIPS/eip-4337.md
abstract contract HSTP is Router {
    constructor(string memory name) {
        register(name, this);
    }

    function query(bytes memory payload)
        public
        view
        virtual
        returns (Response memory);

    function mutation(bytes memory payload)
        public
        payable
        virtual
        returns (Response memory);
}

      HSTP  HSTP
    /    \   /  \
  HSTP   HSTP  HSTP
        /    \
      HSTP  HSTP
    /       /
    HSTP   HSTP

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.0;

import "hstp/HSTP.sol";

// Stateless Hyper Service Transfer Protocol for on-chain services.
contract Todo is HSTP("Todo") {

    struct ITodo {
        string todo;
    }
    
    function addTodo(ITodo memory request) public payable returns(Response memory response) {
        response.body = request.todo;
        return response;
    }

    // Override for HSTP.
    function query(bytes memory payload)
        public
        view
        virtual
        override
        returns (Response memory) {}

    function mutation(bytes memory payload)
        public
        payable
        virtual
        override
        returns (Response memory) {
            (ITodo memory request) = abi.decode(payload, (ITodo));
            return this.addTodo(request);
        }
}

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.0;

import "hstp/HSTP.sol";

// Stateless Hyper Service Transfer Protocol for on-chain services.
contract Todo is HSTP("Todo") {

    struct TodoRequest {
        string todo;
    }
    
    function addTodo(TodoRequest memory request) public payable returns(Response memory response) {
        response.body = request.todo;
        return response;
    }

    // Override for HSTP.
    function query(bytes memory payload)
        public
        view
        virtual
        override
        returns (Response memory) {}

    function mutation(bytes memory payload)
        public
        payable
        virtual
        override
        returns (Response memory) {
            (TodoRequest memory todoRequest) = abi.decode(payload, (TodoRequest));
            return this.addTodo(todoRequest);
        }
}