1. A method in a computing system for augmenting software, comprising: receiving in a target computer system an augmentation specification specifying (a) a programmatic function to be augmented, the specified programmatic function being among software existing in the computing system, (b) an argument of the specified programmatic function, (c) a test to apply to the specified argument, and (d) a modification to perform to the behavior of the specified programmatic function if the specified test is not satisfied by the specified argument; and when the specified programmatic function is invoked on the target computer system, if the specified test is not satisfied by the specified argument, performing the specified modification to the behavior of the specified programmatic function.