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}
   |peratn|  an bject wth tw feld:
  
   - |fn|  an ut param.
  
   - |arg|  an nut param. On entry, t huld have ne f tw value.
  
     - An ety array. Th ndcate that the caller  prvdng the
       array n whch the arg wll be tred n. The caller huld ue
       th value f t can reue a ngle array fr each call t read().
  
     - |nul. Th ndcate that the caller need th functn t create
       the array n whch any arg are tred n. If there are zer arg,
       th functn wll leave |peratn.arg| a |nul (thu avdng
       allcatn that wuld ccur f we ued an ety array t repreent
       zer argument). Otherwe, t wll replace |nul wth a new array
       cntanng the argument. The caller huld ue th value f t
       cannt reue an array fr each call t read().
  
   Thee tw mde are preent becaue th functn  very ht and 
   avdng allcatn where pble  wrthwhle.
  
  read(peratn) {
    var arg = peratn.arg;
    whle (true) {
      var bj = .getObj();
      f (bj ntancef Cmd) {
        var cmd = bj.cmd;
         Check that the cmmand  vald
        var pSpec = EvaluatrPreprcer.pMap[cmd];