Checking whether an object in space is an isosceles triangle

See Section 26.15.6 for checking for isosceles triangles in 2D geometry.

The is_isosceles command determines whether or not a geometric object is an isoceles triangle.

is_isosceles takes G, a geometric object or a sequence of three points assumed to be the vertices of a triangle.
is_isosceles(G) returns 1,2 or 3 if the object is an isoceles triangle (the number indicates which vertex is on two equal sides), returns 4 if the object is an equilateral triangle, and returns 0 otherwise.

is_isosceles([2,0,0],[0,0,0],[0,2,0])

T:=triangle_isosceles([0,0,0],[2,2,0],[2,2,2]); is_isosceles(T)

is_isosceles([1,1,0],[-1,1,0],[-1,0,0])