See section 12.13.9 for checking for equilateral triangles in three-dimensional geometry.
The is_equilateral command is a Boolean function which takes as argument a geometric object or three points.
is_equilateral returns 1 if the object (or the
triangle formed by the three points) is an equilateral triangle and
returns 0 otherwise.
Note that T is a triangle since T is a list made
of a triangle and the vertex C. Entering affix(C)
returns 1 + i*sqrt(3)