function doTransfer(
  address _from,
  address _to,
  uint256 _amount
) internal returns (bool) {
  if (_amount == 0) {
    return true;
  }

  if (parentSnapShotBlock >= block.number) throw;

  // Do not allow transfer to 0x0 or the token contract itself
  if ((_to == 0) || (_to == address(this))) throw;

  // If the amount being transfered is more than the balance of the
  //  account the transfer returns false
  var previousBalanceFrom = balanceOfAt(_from, block.number);
  if (previousBalanceFrom < _amount) {
    return false;
  }

  // Alerts the token controller of the transfer
  if (isContract(controller)) {
    if (!TokenController(controller).onTransfer(_from, _to, _amount)) throw;
  }

  // First update the balance array with the new value for the address
  //  sending the tokens
  updateValueAtNow(balances[_from], previousBalanceFrom.sub(_amount));

  // Then update the balance array with the new value for the address
  //  receiving the tokens
  var previousBalanceTo = balanceOfAt(_to, block.number);
  updateValueAtNow(balances[_to], previousBalanceTo.add(_amount));

  // An event to make the transfer easy to find on the blockchain
  Transfer(_from, _to, _amount);

  return true;
}