%!

/intersection { 1 dict begin
pop
[[-1 -1][0 1]]
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 [1 1 1] 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