Gemara on page 18 and 19 of Bava Kama. A difference between Rambam and Tosphot.

I think that the Rambam holds that there can not be three kinds of payment for pebbles, and that that is how he understands the Gemara on page 18 and 19 of Bava Kama. I mean that there can be half damage and a fourth; or a half and a whole, but not all three. To explain what I mean let me bring the Gemara on page 19. Rav Ashi asked if there can be change for pebbles to bring down the payment from 1/2 to 1/4. The Gemara suggests that we can know this from the question of Rava if there can be warning for pebbles? The Gemara says since this is a question to Rava, therefore there cannot be change to a 1/4. (This is a problem because Rav ashi had a doubt if change is applicable. If the answer is simple he would not have had a question ). so Gemara then suggests that perhaps Rava meant to say that if there is no change, then there must be a question if warning can be applied. (That Is to say that it is not that there is no change for sure. Rather there might be change, and therefore we have no question about warning. Or there might be no change and therefore there is a question about warning.) This Gemara can be summed up thus. If there is a question about warning, then change is not applicable. I.e. the question about warning causes there to be no change. Then the Gemara turns that around, and says if there is no change, there has to be a question about warning. That means to say that the arrow of causation is turned around. But at any rate, the Rambam is first bothered by the same question that bothers Tosphot. How can there be three kinds of payment, 1/4, 1/2 and a whole. The Rambam that you can have three kinds of payment, but only two kinds for every tana (teacher). R Elazar holds there is 1/2 with no warning, and that goes up to full payment when there is warning, (and change does not matter at all, whether it is present or not). The sages hold there is 1/2 payment, and if there is change, that goes down to ¼, (and warning does not apply at all, whether warning was given or not). One thing still bothers me about the conclusion of this Gemara is this: how would the fact that there is no change cause there to be a question about warning? I would like to add one more idea here. Rav Shach brings up a question on the Rambam. Why does he bring the question about change but not the question about warning? I would like to suggest a possible answer to this question. I think the Rambam holds that in our Gemara, we can see that change and warning are incompatible variables. If there is change going down to 1/4, there cannot be warning. If there is warning going up to full payment, there cannot be change going down to a 1/4. Therefore, he brought only the question about change because the law is not like R Elazar who holds that with warning the payment goes up to full payment. (R Elazar holds this in the situation in the mishna about the dog with the loaf and burning coal. He holds this requires full payment when there is warning given three times). But the Rambam decided the law is like the sages that that case required only half payment. But Rav Ashi asked according to the sage if change is possible that would bring the payment down to a fourth. The Rambam therefore decided that change for pebbles is a doubt and leave out the possibility of a warning bring up the payment to full damage because the law is not like R. Elazar. Tosphot however holds that the explanation of our Gemara on page 19 is different. He holds that the meaning of thee Gemara is this. If warning is applicable to pebbles (so that the payment would be full damage), then it would also be applicable to a change in pebbles. That is to say there is change in pebbles that bring it down to a fourth and that warning is applicable in such a way that would bring it back up to a half. (It would not bring to full damage because that would be too much of a jump as Tosphot said before. Tosphot also holds that the gemara holds that if warning is not applicable to pebbles (to bring payment to full damage), then warning cannot be applicable to change either. That is at this point there might be change, but warning would not apply to it to bring it to half payment. Or there might not be change at all. That is even if change happen the payment is till 1/2