 In some circumstances, obligations may be void for uncertainty if they are "pacta de contrahendo", or because they use vague language or are of indefinite duration.The parties may agree on a mechanism for determining what has to be performed.Where this mechanism takes the form of a power granted to a third party, or possibly even to one of the parties to determine what has to be performed, the courts will (depending on the type of contract) uphold the contract, provided that the power has been exercised reasonably.