-6 4 setyrange

/A {-5 -2} def
/B {-3 2} def
/C {-1 -4} def

%% la rotation : centre et angle
/I {-2 1} def
/alpha 100 def

%% la transformation
/T {I alpha rotatepoint} def
I point

[A B C] {T} papply               %% on calcule les images
[/A' /B' /C'] exch mapnp         %% et on les nomme

gsave
   pointilles
   I B B' arcp
   I A A' arcp
   I C C' arcp
grestore

1.2 setcurvelinewidth
[A B C] polygone
[A' B' C'] polygone

bleu
setTimesItalic
   (A) A dltext
   (B) B ultext
   (C) C dltext
   (A') A' rtext
   (B') B' dltext
   (C') C' ultext
   (I) I urtext