contract StandardToken is ERC20, BasicToken {
  mapping (address => mapping (address => uint256)) internal allowed;

  // implementation transfer from another user
  function transferFrom(address _from, address _to, uint256 _value) public returns (bool) {
    require(_value > 0);
    require(_to != address(0));
    require(_value <= balances[_from]);
    require(_value <= allowed[_from][msg.sender]);

    balances[_from] = balances[_from].sub(_value);
    balances[_to] = balances[_to].add(_value);
    allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
    emit Transfer(_from, _to, _value);
    return true;
  }

  // implementation approval to allowed transfer
  function approve(address _spender, uint256 _value) public returns (bool) {
    require(_value > 0);
    allowed[msg.sender][_spender] = _value;
    emit Approval(msg.sender, _spender, _value);
    return true;
  }

  // check total allowance particular user
  function allowance(address _owner, address _spender) public view returns (uint256) {
    return allowed[_owner][_spender];
  }

 // implementation increase total approval from particular user
  function increaseApproval(address _spender, uint _addedValue) public returns (bool) {
    require(_addedValue > 0);
    allowed[msg.sender][_spender] = allowed[msg.sender][_spender].add(_addedValue);
    emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
    return true;
  }

  // implementation decrease total approval from particular user
  function decreaseApproval(address _spender, uint _subtractedValue) public returns (bool) {
    require(_subtractedValue > 0);
    uint oldValue = allowed[msg.sender][_spender];
    if (_subtractedValue > oldValue) {
      allowed[msg.sender][_spender] = 0;
    } else {
      allowed[msg.sender][_spender] = oldValue.sub(_subtractedValue);
    }
    emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
    return true;
  }

}
function approve(address _spender, uint256 _value) public returns (bool) {
  require(_value > 0);
  allowed[msg.sender][_spender] = _value;
  emit Approval(msg.sender, _spender, _value);
  return true;
}

// implementation approval to allowed transfer