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It frequently happens, especially with enduring contracts, that either or both contracting parties wish to change the contract as they go. Where the contract itself does not provide a procedure for making variations (or, if it does, the parties for whatever reason omit to follow that procedure), the parties can always vary their contract informally. Informal variations, however, can generate some thorny legal issues, especially if the variation privileges one party only (for example, the variation is that the contractor will be paid more simply to finish what the origial contract already requires of him), or if the promisor assented to the variation because the promisee had the promisor 'over a barrel,' so to speak. To be sure, a number of technical legal rules potentially come into play in post-contractual modification situations. Two such rules are the 'pre-existing duty rule,' inside contract law's consideration doctrine, and 'economic duress.'
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