14.13.11 Checking if an object in space is a right triangle or a rectangle: is_rectangle
See Section 13.16.7 for checking for right triangles and
rectangles in two-dimensional geometry.
The is_rectangle command determines whether or not a
geometric object is an rectangle or a right triangle.
is_rectangle takes one argument: G, a geometric object or a sequence of three or four points
assumed to be the vertices of a triangle or a quadrilateral.
is_rectangle(G) returns:
(for triangle G) 1,2 or 3 if G is a right
triangle (the number indicates which vertex is has the right angle).
(for quadrilaterals G) 1 if G is a rectangle but not a
square.