code: func (l *Line) Direction() float64 {
  	return math.Atan2(l.b[1]-l.a[1], l.b[0]-l.a[0])
  }
docs: // Direction computes the direction the line is pointing from A() to B().
  // The units are radians from the positive x-axis.
  // Range same as math.Atan2, [-Pi, Pi]