feat:soul gas token

ecosystem/soul-gas-token.md

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+# Purpose
+
+The purpose of this document is to introduce the rationale and proposed design for `soul gas token`(`SGT`), a *
+*non-transferable** wrapped L2 native gas token. It provides a new way to subsidize and incentivize users, trying to
+solve the problems of the current way.
+
+# Problem Statement + Context
+
+To promote mass adoption of web3 and draw in users from web2, project teams often apply strategies like fee subsidies
+and airdrops.
+
+- The practice of using fee subsidies typically involves employing account abstraction through EIP-4337, yet adopting
+  smart contract wallets continues to pose difficulties. Despite ongoing discussions about EIP-3074 and EIP-4337, most
+  users continue to prefer traditional wallets.
+- Airdrops have not been effective in maintaining genuine user engagement, as there is a noticeable decline in on-chain
+  activity post-redemption.
+
+To tackle the challenges faced by chain operators, `SGT` is proposed as an innovative approach to subsidize and motivate
+users.
+
+# Proposed Solution
+
+We introduce `SGT`, an ERC-20 token that mirrors the L2 native gas token with a 1:1 value peg.
+SGT permits the L2 native gas token to act as collateral and serve as the gas token on L2, though it is not transferable
+between users.
+Both Chain operators and Governance foundations can leverage `SGT` to incentivize users, who in turn can utilize `SGT`
+for paying transaction fees.
+
+## Prerequisites
+
+The prerequisite for using `SGT` is to use the [
+`custom gas token`](https://specs.optimism.io/experimental/custom-gas-token.html) feature of the OP Stack as the native
+gas token for L2.
+
+## Implementation
+
+### Contract
+
+#### Non-transferable feature
+
+Implement the non-transferable feature. Disallow the functions below:
+
+```solidity
+
+    function transfer(address _to, uint256 _value) public returns (bool success);
+
+    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success);
+
+    function approve(address _spender, uint256 _value) public returns (bool success);
+
+    function allowance(address _owner, address _spender) public view returns (uint256 remaining);
+
+```
+
+#### Deposit and withdrawal
+
+Implement the deposit and withdrawal functions, inheriting
+the [ERC20Wrapper](https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/extensions/ERC20Wrapper.sol)
+and overriding
+the `deposit` and `withdraw` functions. Chain operators or governance can mint and burn `SGT` by depositing and
+withdrawing the L2 native gas token.
+
+### State Transition Logic
+
+#### Gas fee processing 
+
+Modify the state transition logic to utilize `SGT` for gas fees. 
+
+1. Adjust the `busGas` function logic: if the `SGT` contract is already deployed, in addition to fetching the user's balance, the contract's `balanceOf` method should be invoked to retrieve the user's `SGT` balance. Prioritize using `SGT` to purchase `Gas` first.
+```go
+ func (st *StateTransition) buyGas() error
+```
+
+2. Refund the user's `SGT` if it remains after the gas fee is paid.
+
+3. Fee Rewards also uses `SGT` to reward sequencer.
+
+
+# Challenges
+
+1. How to reward fees to sequencers using `SGT`? 
+- The functions of transferring `SGT` are disabled, so how can sequencers receive `SGT` as rewards?
+- How to ensure that the 1:1 peg between `SGT` and the L2 native gas token is maintained? 
+
+# Security Considerations
+
+1. Risk of DoS Attacks
+2. Sybil Attack Prevention