- 
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)