autocrop
-10 4 setyrange
-15 5 setxrange

/A {-2 2} def 
/B {2 2} def 
/C {2 -2} def 
/D {-2 -2} def 
D point

[A B C D] polygone
[A C] ligne
[B D] ligne

/C1 {D D A distance} def
/C2 {C A C distance} def
/E {-90 C1 cpoint} def

90 270 C1 Cercle
135 225 C2 Cercle

gsave
   newpath
      E smoveto
      270 90 C1 Cercle_
      135 225 C2 Cercle_
   clip
   hachure
grestore

pointilles
[D E] ligne

setTimesItalic
   (A) A uctext
   (B) B urtext
   (C) C drtext
   (D) D drtext
   (E) E drtext

<tex>
${\cal C}_1$
</tex>
130 C1 cpoint [1.3 dup] ultexlabel

<tex>
${\cal C}_2$
</tex>
170 C2 cpoint [1.3 dup] drtexlabel