Rabbi Judah used to abbreviate the plagues: Rabbi Judah wished to show that in terms of justice, one has to receive a warning (in order to be held culpable for breaking the law) so that one did not do so out of ignorance. But we do not find a warning to Pharaoh before the plagues of lice, boils and darkness. Also, there was no warning before all of the plagues that they would strike Egypt. Rabbi Judah groups the plagues in this way to show that there was a warning before each grouping, D'tzakh, Adash and Ba'akhav. These three groups were inscribed on Moses' staff.