XPreadEnv()
  Read an environment string from the original Env.
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  Function:    XPreadEnv()

               Just like Clipper's GetEnv(), but changes made with XPsetEnv()
               are correctly refelected here.

  Syntax:      XPreadEnv(cVar)

  Arguments:   <cVar> is the variable one wishes to read.

  Returns:     The Environment's variable value, or "" when
               it is not found.

  Usage:       * Assuming PROMPT is $p$g
               XPsetEnv('PROMPT','(Shell) '+GetEnv('PROMPT'))
               ? XPreadEnv('PROMPT')  && Returns "(Shell) $p$g"
               ? GetEnv('PROMPT')   && Returns "$p$g"