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Our tree is actually a tree of the short-term interest rate. The average direction in which the short-term interest rate moves depends on the level of the rate. When the rate is very high, that direction is downward; when the rate is very low, it is upward.
我们的树实际上是短期利率树。短期利率变动的平均方向取决于利率水平。当利率很高时，这个方向是向下的;当利率非常低时，它是上升的。