unperm := proc(av) local a, n, b, i, j; 
a := av; n := nops(a); b := NULL; 
for i to n do 
if a[i] <> i then
 for j from i + 1 to n do
     if a[j] = i then break;
     end if; 
 end do; 
   if n < j then 
      error "geen perm"; 
    end if;
 a[j] := a[i]; 
 a[i] := i; 
 b := b, [i, j]; 
 end if; 
end do; 
[b]; 
end proc;


unperm([1, 2, 3]);
                               []
unperm([1, 3, 2]);
                            [[2, 3]]
unperm([4, 5, 2, 1, 3]);
                    [[1, 4], [2, 3], [3, 5]]