See section 12.6.2 for perpendicular bisectors in space.
The perpen_bisector command takes as argument a line segment or two points representing the end points of a line segment.
perpen_bisector returns and draws the perpendicular bisector
of the segment.
The perpen_bisector command can also take two lines as segments, in which case it returns and draws the perpendicular bisector of the segment from the first point defining the first line and the second point defining the second line.