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Compilers to "optimize" the code produced from computer programs have been in use for many years. Optimization improves execution time of a program by changing or moving some of the statements and leaving the correctness of the program intact. All crucial relationships are maintained between the variables. Computations are tests are altered to reduce the time needed for computation. One technique of optimization imvolves various program transformations that are independent of the machine code. They can be applied to an intermediate-level program prior to machine code generation or directly to source-level programs. The user defines the acceptable input values for the program and the desired relationship between the input and output. "Invariant assertions" are then attached to prechosen points in the program. The correctness of the program with respect to its specifications is provided by these assertions. Figures.