docs: public boolean setExpr(String str){
  		Loc loc=new Loc();
  		loc.v=0;
  		int num1,num2,op;
  		num1=chn2Num(str,loc);
  		if(num1<0)
  			return false;
  		op=chn2Op(str,loc);
  		if(op<0)
  			return false;
  		num2=chn2Num(str,loc);
  		if(num2<0)
  			return false;
  		if(!this.checkSuffix(str, loc))
  			return false;
  		this.num1=num1;
  		this.num2=num2;
  		this.op=op;
  		return true;
  	}