See also section 11.11.1
The arc command takes three mandatory arguments and one or two optional arguments. The mandatory arguments are two points A and B and a real number a between −2π and 2π.
arc returns and draws the circular arc from A to
B that represents and angle of a.
(Note that the center of the circle will be
(A + B)/2 + i*(B - A)/(2*tan(a/2)).)
The optional arguments need to be variable names which will be assigned to the center and the radius of the circle.