This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
Whereas, the breach of the foregoing covenant will cause a serious and substantial damage to the said A. B., and whereas, it would be difficult, and in most cases impossible, to prove the amount of such damage, the said C. D. hereby covenants that in case of breach of the foregoing covenant on his part, he will pay to the said A. B. the sum of --------- dollars, within --------- days, after said breach; such sum being agreed upon by the parties as the amount to which the said A. B. will be damaged by the breach of said covenant on the part of C. D.