code: public AmortizedPQueue<E> minus() {
  		if (size() == 0) {
  			return this;
  		}
  		
  		int fsize = front.size();
  		
  		if (fsize == 0) {
  			//If there's nothing on front, dump back onto front 
  			//(as stacks, this goes in reverse like we want)
  			//and take one off.
  			return new AmortizedPQueue<E>(Empty.<E>stack().plusAll(back), Empty.<E>stack()).minus();
  		} else if (fsize == 1) {
  			//If there's one element on front, dump back onto front,
  			//but now we've already removed the head.
  			return new AmortizedPQueue<E>(Empty.<E>stack().plusAll(back), Empty.<E>stack());
  		} else {
  			//If there's more than one on front, we pop one off.
  			return new AmortizedPQueue<E>(front.minus(0), back);
  		}
  	}
docs: /* Amortized O(1), worst-case O(n)