Use of virus functions to provide a virtual APL interpreter under user control

By inserting a simple statement at the beginning of an APL function, it is possible to cause the execution of a reconstructed version of the function, rather than of its original statements. This reconstruction can simulate the effect of an arbitrary change in the APL language; thus, the APL programmer can have available a private, “virtual” version of the interpreter We show some general- and special - purpose functions which can be used for this purpose, and give elementary examples of their application.