Baba Kama 44b - Hasra'as Safeik

The gemara says that in a case where one throws a stone into an area that has half yisroelim and half goyim, he is patur from capital punishment because "safeik nefashos l'hakel". Tosafos asks, why do we need the concept of "safeik nefashos l'hakel", even without that he should be patur since it is a warning of safeik. Since it is not clear that this action he is about to do will kill a yisroel, it is a hasra'as safeik which is not called a warning at all, so he should be exempt since he wasn't warned? Tosafos answers that really the gemara only works out according to the opinion who says that even a warning m'safeik qualifies as a warning, and makes a distinction between this and another case. However, the Rashba explains that whenever we have a majority, since the rule is that we assume the stone will hit the majority, it qualifies as a definitive warning. If there were 9 goyim and 1 yisroel, we would not say it is a hasra'as safeik, rather it is like all goyim. Similarly, if there were 9 yisroelim and 1 goy, we consider it a definite warning since we assume the rock will hit a yisroel, just that the torah exempts him based on the concept fo ka'vuah. The concept of kavu'ah is that they are treated as half and half, where we then implement the pasuk of "v'hitzilu ha'eida" to say safeik nefashos l'hakel, which makes it as if it were mostly goyim. Basically, so long as there is a rov in one direction or another, we consider it a definite hasra'ah as if he were warned not to kill whoever is the rov. When it is half and half, or kavuah, we use the concept of safeik nefashos l'hakel to make it as if it were mostly goyim, to the exclusion of being like mostly yisroel (which would then be a definite warning for yisroel to make him chayev misah).