Basically the same as drive(), but omitting the right motor.

def left_motor(self, speed, durationS=-1.0):
        """Basically the same as drive(), but omitting the right motor."""
        if speed > 0:
            self.left_rev(min(abs(speed), 255))
        else:
            self.left_fwd(min(abs(speed), 255))
        if durationS > 0:
            self.board.sleep(durationS)
            self.left_stop()