Explain the Ownable contract pattern and provide a simple example of its usage.

Ownable is a basic access control pattern that provides a single owner who has exclusive rights to perform certain administrative functions. It's one of the simplest forms of access control in smart contracts.

Key features:

• Single owner address
• onlyOwner modifier for restricted functions
• Ability to transfer ownership
• Ability to renounce ownership

import "@openzeppelin/contracts/access/Ownable.sol";

contract MyContract is Ownable {
    uint256 public value;
    
    constructor() Ownable(msg.sender) {}
    
    function setValue(uint256 _value) public onlyOwner {
        value = _value;
    }
    
    function emergencyStop() public onlyOwner {
        // Only owner can call this
    }
}

The onlyOwner modifier ensures only the contract owner can call these functions, providing basic administrative control.