%!

/intersection { 1 dict begin
/L exch def
/A L 0 get def
/B L 1 get def
/C L 2 get def
/f00 A -1 mul B -1 mul add C add def
/f10 A  1 mul B -1 mul add C add def
/f11 A  1 mul B  1 mul add C add def
/f01 A -1 mul B  1 mul add C add def

% f01 f11
% f00 f10

[
  f00 f10 mul 0 lt {
    [0 -1]
  } if
  f10 0 eq {
    [1 -1]
  } if
  f10 f11 mul 0 lt {
    [0 1]
  } if
  f11 0 eq {
    [1 1] 
  } if
  f11 f01 mul 0 lt {
    [0 1]
  } if
  f01 0 eq {
    [-1 1] 
  } if
  f01 f00 mul 0 lt {
    [-1 0]
  } if
  f00 0 eq {
    [-1 -1] 
  } if
]
end } def

% --- sample usage ---------------------------------------

/S 72 def
S dup scale
1 S div setlinewidth

4.25 5.5 translate

newpath
-1 -1 moveto
1 -1 lineto
1 1 lineto
-1 1 lineto
closepath
stroke

/I [3 -1 2] intersection def

I length 2 eq {
  newpath
    I 0 get 0 get
    I 0 get 1 get
  moveto
    I 1 get 0 get
    I 1 get 1 get
  lineto
  stroke
} if