The Erfurt massacre occurred at the Gutenberg Gymnasium in Erfurt, Thuringia, Germany, on April 26, 2002. Thirteen teachers, two students and a police officer were shot dead by an expelled student, age 19, who then killed himself, presumably after being confronted by one of his teachers.
... because it discriminates between three qualitatively different cases:
If the discriminant is zero, then there is a repeated solution x, and this solution is real. ...