You can only hold a keen dog back,because it has the desire and pray drive to be held back in the first place, the dog that has spewed has never had that init to start with , but if it has been to ground and took to much pain as a youngster and then spewed then it might come back but normally if a dog hearts is not 100% in the job, you will never fully trust the dog and in the back of your mind there will be doubt! and if your only running one terrier that's not much good , if you cull the dog then it can never prove you wrong . But only the terrier man who's dog this can knows and should be