contract StandardToken is ERC20{
  using SafeMath for uint256;

    mapping(address => uint256) balances;
    mapping (address => mapping (address => uint256)) allowed;

  
    function _transfer(address _from, address _to, uint256 _value) internal returns (bool success){
      //prevent sending of tokens from genesis address or to self
      require(_from != address(0) && _from != _to);
      require(_to != address(0));
      //subtract tokens from the sender on transfer
      balances[_from] = balances[_from].safeSub(_value);
      //add tokens to the receiver on reception
      balances[_to] = balances[_to].safeAdd(_value);
      return true;
    }

  function transfer(address _to, uint256 _value) public returns (bool success) 
  { 
    require(_value <= balances[msg.sender]);
      _transfer(msg.sender,_to,_value);
      emit Transfer(msg.sender, _to, _value);
      return true;
  }

  function transferFrom(address _from, address _to, uint256 _value) public returns (bool) {
      uint256 _allowance = allowed[_from][msg.sender];
      //value must be less than allowed value
      require(_value <= _allowance);
      //balance of sender + token value transferred by sender must be greater than balance of sender
      require(balances[_to] + _value > balances[_to]);
      //call transfer function
      _transfer(_from,_to,_value);
      //subtract the amount allowed to the sender 
      allowed[_from][msg.sender] = _allowance.safeSub(_value);
      //trigger Transfer event
      emit Transfer(_from, _to, _value);
      return true;
    }

    function balanceOf(address _owner) public constant returns (uint balance) {
      return balances[_owner];
    }

    

  /**
   * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
   *
   * Beware that changing an allowance with this method brings the risk that someone may use both the old
   * and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
   * race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
   * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
   * @param _spender The address which will spend the funds.
   * @param _value The amount of tokens to be spent.
   */

  function approve(address _spender, uint256 _value) public returns (bool) {
    // To change the approve amount you first have to reduce the addresses`
    //  allowance to zero by calling `approve(_spender,0)` if it is not
    //  already 0 to mitigate the race condition described here:
    //  https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
    require((_value == 0) || (allowed[msg.sender][_spender] == 0));
    allowed[msg.sender][_spender] = _value;
    emit Approval(msg.sender, _spender, _value);
    return true;
  }

  /**
   * @dev Function to check the amount of tokens that an owner allowed to a spender.
   * @param _owner address The address which owns the funds.
   * @param _spender address The address which will spend the funds.
   * @return A uint256 specifying the amount of tokens still available for the spender.
   */
  function allowance(address _owner, address _spender) public view returns (uint256) {
    return allowed[_owner][_spender];
  }

}
function allowance(address _owner, address _spender) public view returns (uint256) {
  return allowed[_owner][_spender];
}

/**
 * @dev Function to check the amount of tokens that an owner allowed to a spender.
 * @param _owner address The address which owns the funds.
 * @param _spender address The address which will spend the funds.
 * @return A uint256 specifying the amount of tokens still available for the spender.
 */