// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../ERC20.sol";

abstract contract ERC20Snapshot is ERC20 {

  mapping(uint256 => mapping(address => uint256)) private _snapshots;

  uint256 private _currentSnapshotId;

  event Snapshot(uint256 id);

  // Create a new snapshot.

  function _snapshot() internal returns (uint256) {

    _currentSnapshotId++;

    emit Snapshot(_currentSnapshotId);

    return _currentSnapshotId;

  }

  // Retrieve balance at a given snapshot.

  function balanceOfAt(address account, uint256 snapshotId) public view returns (uint256) {

    return _snapshots[snapshotId][account];

  }

  // Logic to update snapshots on transfers is omitted for brevity.

}