### 13.12.9 The measure of an angle in space: angle

See section 12.14.4 for angle measures in two-dimensional geometry.

The angle command takes are arguments one of the following;

- Three points, A, B and C.
  angle returns the measure of the undirected angle BAC.
  Input:
  angle(point(0,0,0),point(1,0,0),point(0,0,1))
  Output:
  pi/4
- Two intersecting lines.
  angle returns the measure of the angle between the lines.
  Input:
  angle(line([0,0,0],[1,1,0]),line([0,0,0],[1,1,1]))
  Output:
  acos((sqrt(6))/3)
- A line and a plane.
  angle returns the measure of the angle between the line and the plane.
  Input:
  angle(line([0,0,0],[1,1,0]),plane(x+y+z=0))
  Output:
  acos((sqrt(6))/3)