String invert() {
	    if (canInvert) {
	        cleanX = true;
	        cleanY = true;
	        endZero = "";
	        if (enterX) {
	            x = 1 / x;
	            return format(x);
	        } else if (! canCalculate) {
	            x = 1 / x;
	            return format(x);
	        } else {
	            y = 1 / y;
	            return format(y);
	        }
	    }
	    else return format(result);
	}