This Excel spreadsheet calculates the price of a Bond option with a binomial tree.
Bond options give the purchaser the right (but not the obligation) to buy or sell a bond at or before a specific date.
If you purchase a bond call, you generally expect interest rates to decrease (with a subsequent increase the price of a bond).
However, if you purchase a bond put, you generally expect interest rates to climb, and bond prices to fall.
Other spreadsheets use the Black 76 and the Shaefer & Schwartz models to price bond options.