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pragma solidity^0.4.16;

library SafeMath {
  function mul(uint256 a, uint256 b) internal constant returns (uint256) {
    uint256 c = a * b;
    assert(a == 0 || c / a == b);
    return c;
  }
  function div(uint256 a, uint256 b) internal constant returns (uint256) {

    uint256 c = a / b;
    return c;
  }
 
  function sub(uint256 a, uint256 b) internal constant returns (uint256) {
    assert(b <= a);
    return a - b;
  }
 
  function add(uint256 a, uint256 b) internal constant returns (uint256) {
    uint256 c = a + b;
    assert(c >= a);
    return c;
  }
}

contract TestOVerflow{
    using SafeMath for uint256;
    function add(uint8 a, uint8 b) public returns(uint8){
        return a + b;
    }
    
    function test_uint256(uint256 a) public returns(uint256){
        return a*2;
    } 
    function test_uint256_safemath(uint256 a) public returns(uint256){
        return a.mul(2);
        
    }
}