autocrop
/A {1 0} def
/B {1 1} def
/C {0 1} def

-90 A 1 cpoint /A' defpoint
0 B 2 cpoint /B' defpoint
90 C 3 cpoint /C' defpoint
180 O 4 cpoint /O' defpoint

gsave
newpath
   .8 setgray
   O smoveto
   -180 -90 A 1 Cercle_
   -90 0 B 2 Cercle_
   0 90 C 3 Cercle_
   90 180 O 4 Cercle_
fill
grestore

[O A B C] polygone
O A 1 marked
A B 1 marked
B C 1 marked
C O 1 marked

gsave
   pointilles
   [A A'] ligne
   [B B'] ligne
   [C C'] ligne
   [O O'] ligne
grestore

1 setlinewidth
-180 -90 A 1 Cercle
-90 0 B 2 Cercle
0 90 C 3 Cercle
90 180 O 4 Cercle

10 setfontsize
setTimesItalic
   (x) O C milieu (-3 0) cltext