%autocrop
20 setxunit
%quadrillage marks

-10 9 setxrange
-5.5 12 setyrange

%% le plan de base
/P1 {-9 -3} def
/P2 {4 -5} def
/P3 {8.5 0} def

/a {-6 6 0} def    
/b {6 6 0} def    
/c {6 -6 0} def    
/d {-6 -6 0} def    
/s {0 0 11.4} def
/s' {s pop 0} def

/alpha 35 def
/beta 170 def
/vect_I {alpha cos alpha sin .5 mulv} def
/vect_J {beta cos beta sin .9 mulv} def
/vect_K {0 1} def

%% [O vect_I] (->) ligne
%% [O vect_J] (->) ligne
%% [O vect_K] (->) ligne

/xyz2xy {
3 dict begin
   /z exch def
   /y exch def
   /x exch def
   vect_I x mulv
   vect_J y mulv
   vect_K z mulv
   addv addv
end
} def

[/A /B /C /D /S /S'] 
[a b c d s s'] {xyz2xy} capply
mapnp

%% corrige
A B milieu /I defpoint
A D milieu /J defpoint
B D J C interdroite /K defpoint
S S' K paral S D interdroite /K' defpoint

[J I K K'] {times2} plot
/fillstyle {.9 setgray fill} def
gsave
   .6 setlinewidth
   mixte
   [J K' C] polygone*
   .8 setlinewidth
   orange
   [I D B] ligne
   noir
   pointilles
   [K K'] ligne
grestore
%%%%%%%%%%

/dotscale {2 dup} def
[S'] {times} plot

1.2 setlinewidth
[A D C S] ligne
[A S D] ligne

gsave
   .8 setlinewidth
   pointilles
   [A B C] ligne
   [B S S'] ligne
grestore

2 setlinewidth
[-12 P2 P1 xdpoint P2 P3] ligne


12 setfontsize
setTimesItalic
   (A) A dltext   
   (B) B ultext	  
   (C) C urtext	  
   (D) D drtext	  
   (I) I (3 0) dctext	  
   (S) S ultext   
   (T) K' urtext   
   (s) S' brtext   
   (H) P2 (0 7) ultext