function _preValidatePurchase(
    address _beneficiary,
    uint256 _weiAmount
  )
  internal
  {
    require(_weiAmount >= minimumContribution);
    require(
      contributions.weiContributions(_beneficiary).add(_weiAmount) <= maximumContribution
    );
    super._preValidatePurchase(_beneficiary, _weiAmount);
  }