1. An electronic digital computer which is programmed with an operating system comprising: a main module configured to process current system calls; an emulator module configured to interpret a running retrograde program, the retrograde program being configured to generate current system calls and retrograde system calls, the emulator module being further configured to process the retrograde system calls; a main call gate to which current system calls and retrograde system calls generated by the retrograde program are directed, the main call gate pointing to the emulator module; and an alternate call gate which points to the main module; in which the emulator module is further configured to redirect current system calls to the alternate call gate.