Considerations:
    The Expand library currently sports a number of functions that
    make playing around with the Environment strings possible. Great
    care has to be taken when doing so, as the functions perform unusual
    tasks. The EnvXXXX() have to be approached with care, the SetEnv()
    DelEnv() and ReadEnv() functions, on the other hand, can be used without
    much prior knowledge.

    Undocumented features are used to achieve this, so no guarantee can be
    given that it'll work everywhere (although, to my knowledge, it does
    work, as DOS also uses the same methods currently).

    The EnvXXXX() functions alter the Original Copy of the environment
    strings. Every time one loads a copy of COMMAND.COM, a block with
    environment strings is supplied, which is what you see when you
    type "SET" at DOS. When a program is invoked, a copy of those
    strings is supplied to the program. The EnvXXXX() functions, however,
    alter the contents of the original strings. Changes are thus
    not visible when using Clipper's GetEnv(), but, they could be visible
    to programs executed from the Clipper Application (*). This feature thus
    makes it possible to change the PROMPT, the PATH or even add (or
    delete) some Environment variables for child processes.

    Note that the environment has a maximum size, which one can not exceed.
    Note also that changes made to the environment are also kept when
    the program terminates! Ensure yourself that you restore the
    original enviroment when QUITting..

    The EnvXXXX() perform quite 'Low level' Tasks. The other functions
    use the EnvXXXX() functions to do what they are intended to do.

    Again, please know what you're doing when playing with these functions..
    It might also seem quite difficult to implement it for your application,
    but have a look at EXPDEMO.PRG, which quite well illustrates setting
    a different PROMPT for a Shell from Clipper, a feature much requested
    for Clipper applications.

    (*) You need to call a special function before running to ensure that.