|canvas shape alpha a b m s|
canvas := DrGeoCanvas new fullscreen.
alpha := canvas freeValue: -90 degreesToRadians.
shape := [:c :o :f| | e p |
   "c,o,e: center, origin and extremity of the arc"
   e := canvas rotate: o center: c angle: alpha.
   canvas arcCenter: c from: o to: e.
   p := canvas translate: e vector: (canvas vector: c to: o).
   (canvas polygon: { c. o. p. e }) name: f.
   e].
a := canvas point: 1@0.
b := canvas point: -1 @0.
m := canvas middleOf: a and: b.
s := shape value: m value: a value: 1.
shape value: m value: s value: 1.