mplus报错，应该怎么办呢

Question

运行结果及报错内容

``` TITLE:A lette
  DATA:
    FILE IS D:\CGSS\Z.dat;
  VARIABLE:
    MISSING ARE ALL (-99);
    NAMES ARE ZF1 ZF2 ZC ZSSC ZW ZS;
    USEVARIABLE = ZF1 ZC ZSSC ZW SSCC ZZFC;
  DEFINE:SSCC = ZSSC*ZC;
  ZZFC = ZF1*ZC;
  ANALYSIS:bootstrap = 1000;
  MODEL:ZW ON ZF1(b1)
        ZSSC
        ZC
        ZZFC(b2)
        SSCC;
        ZF1 ON ZSSC(a1)
        ZC
        SSCC(a3);
        ZZFC with ZF1;
  MODEL CONSTRAINT:
        new(ind wmod);
        wmod = -1
        ind = (a1 + a3*wmod)*(b1  +b2*wmod);
  OUTPUT:CINTERVAL(bcbootstrap);

*** ERROR in MODEL CONSTRAINT command
  Only one sign (=, > or <) is allowed for each statement.  Problem with the
  following statement starting with:  WMOD.