This is my code,
I want to know the reason why I am not able to change the value in the variable a.
Would you please give me the reason or any information from solidity doc?

```
// SPDX-License-Identifier: MIT
pragma solidity >=0.8.0 <0.9.0;

contract simple {
    uint public a = 3;
    a = 16; // error occurred : parser Error expected identifier but got '='
    }

```