A circle is internally represented by its center and radius.
circle(A, x)Circle of center A and radius x.
circle(s)Circle of diameter s.
radius(c)Radius of circle c.
perimeter(c)Perimeter of circle c.
area(c)Area enclosed in circle c.
arg(A, c)Polar angle of point A with respect to the center of c (in degrees).
point(c, a)Point on c with polar angle a with respect to its center.
center(c)Center of circle c.
line(c, a)Tangent line to circle c. The contact point has polar angle a with respect to the center of c.