See section 11.10.2 for regular polygons in the plane.
The isopolygon command takes as arguments three points and an integer.
isopolygon returns and draws a regular polygon. The first
two points are adjacent vertices of the polygon, the third determines
the plane and orientation of the polygon. The fourth argument, the
integer, determines the number of sides of the polygon. If the fourth
argument is positive, then it is the number of sides of the polygon,
which is positively oriented. If the fourth argument is negative,
then the polygon is negatively oriented and the absolute value of the
argument is the number of sides.