The env [array] is a [magic name] created by [Tcl] to reflect
the value of the invoker's [environment]; that is to say, when
one starts Tcl, what one does is make a request to the operating system
to start a new process.  If the process is being started directly, that
process is typically given a set of variables called ''environment variables''
which reflect certain default values.  When Tcl starts, it creates the
env array and reads the environment.  Each environment variable becomes
a key in this array whose value is the value of the environment variable.

For instance, on Unix, one will typically find keys like

   * PATH : series of file system directory ''prefixes'', typically seperated by :, which the operating system will apply when asked to execute a file with an incomplete pathname.
   * HOME : name of the user's login directory.
   * LANG : string used to specify internationalization info.