General pattern for algorithms whose recurrence includes only one recursive term. Special cases are:

DecreaseByConstant: T(n) = T(n-c) + f(n).

DecreaseByConstantFactor: T(n) = T(n/c) + f(n).

DecreaseByVariableFactor: T(n) = T(k) + f(n), where k may be a RandomVariable or depend on the problem instance.

Some operations on BinaryTrees.