underscore.sol

pragma solidity ^0.4.18;

contract underscore {
    uint public a;
    constructor() public { 
        a = 10;
    }
    
    modifier conditionalChangeAtoThirty() {
        _;
        if (a == 20){
            a = 30;  
        }
    }
    
    function changeaToTwenty() 
        public 
        conditionalChangeAtoThirty
    {
        a = 20;
    }
}