contract LimitedTransferToken is ERC20 {

  /**
   * @dev Checks whether it can transfer or otherwise throws.
   */
  modifier canTransfer(address _sender, uint256 _value) {
    require(_value <= transferableTokens(_sender, uint64(now)));
   _;
  }

  /**
   * @dev Checks modifier and allows transfer if tokens are not locked.
   * @param _to The address that will receive the tokens.
   * @param _value The amount of tokens to be transferred.
   */
  function transfer(address _to, uint256 _value) canTransfer(msg.sender, _value) public returns (bool) {
    return super.transfer(_to, _value);
  }

  /**
  * @dev Checks modifier and allows transfer if tokens are not locked.
  * @param _from The address that will send the tokens.
  * @param _to The address that will receive the tokens.
  * @param _value The amount of tokens to be transferred.
  */
  function transferFrom(address _from, address _to, uint256 _value) canTransfer(_from, _value) public returns (bool) {
    return super.transferFrom(_from, _to, _value);
  }

  /**
   * @dev Default transferable tokens function returns all tokens for a holder (no limit).
   * @dev Overwriting transferableTokens(address holder, uint64 time) is the way to provide the
   * specific logic for limiting token transferability for a holder over time.
   */
  function transferableTokens(address holder, uint64 time) public constant returns (uint256) {
    return balanceOf(holder);
  }
}
function transfer(address _to, uint256 _value) canTransfer(msg.sender, _value) public returns (bool) {
  return super.transfer(_to, _value);
}

/**
 * @dev Checks modifier and allows transfer if tokens are not locked.
 * @param _to The address that will receive the tokens.
 * @param _value The amount of tokens to be transferred.
 */