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  Name:     f_ndxkey() - get the index expression of an index file
  Usage:    <string> = f_ndxkey(<NTX_NDX_file>)
  Params:   string <NTX_NDX_file> - index filename to check
  Returns:  string equal to the index expression in the index file

 ---------------------------------- Example ---------------------------------

                 use employee
                 index on upper(trim(empname)) to empname
                 close databases
                 memvar = f_ndxkey("empname.ntx")
                 ? memvar  && prints "upper(trim(empname))"

  Note:     F_ndxkey() does not check that the file is a valid
            index file.