Someday you may be faced with a deprecation situation where you need to add a compatibilityswitch to your program. When in this situation it may be tempting toresort to controlling things with an environment variable, instead ofgiving your program yet another command line option.

(The problem with using command line options is that they add clutter,complexity, or both to your documentation, including any usage stuffthat the program prints.)

Unfortunately this is not a good way to add such switches to yourprogram, especially compatibility switches