Clever hack

[109]

Normally such a back door could be removed by removing it from the source code for the compiler and recompiling the compiler. But to recompile the compiler, you have to use the compiler -- so Thompson also arranged that the compiler would recognize when it was compiling a version of itself, and insert into the recompiled compiler the code to insert into the recompiled `login' the code to allow Thompson entry -- and, of course, the code to recognize itself and do the whole thing again the next time around!