George is taking a management class with the biggest jerk of a professor on his campus. This professor wants students to be absolutely quiet during his lectures, so when students first walk into class, they are treated to the music of Lawrence Welk being played over the sound system. In order to get the music to stop playing, all of the students in the class have to be absolutely quiet. Only then does the professor turn off the music and start his lecture. George's professor is using $-$ to control the students in his classroom.