Returns a command line parameter
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 Syntax

        GT_argv(<nParam>) --> cParam

 Arguments:

      <nParam>  -  The number of the parameter on the command line

 Returns:

      <cParam>  -  The parameter asked for

 Description:

      Return a command line parameter
      Reads the C startup variable to find the specified parameter.

      If <nParam> == 0, the function will return the name of the EXE
      including it's path on non-prehistoric DOSes.

 Examples:

      local i

      for i := 0 to GT_Argc()
          ? "Parameter Nº " + str(i, 2) + " = " + GT_Argv(i)
      next
  $CAVEATS$
      Does not check <nParam> is a valid parameter.

 Source: ARGV.C

 Author:  Andy M Leighton