uses Crt;
const
  MaxNum=30;
type
   pEl= ^Element;
   Element= record
              Data:integer;
              Next:pEl;
              Prev:pEl
            end;
var
   PX, PY : pEl;
   firstElem, curElem: pEl;
   i:integer;
   f:TEXT;
{***************************}
procedure AddElem(var CurElem:pEl; val:integer);
var
   tmpElem:pEl;
begin
   New(tmpElem);
   tmpElem^.Data := val;
   tmpElem^.Next := nil;
   tmpElem^.Prev := CurElem;
   CurElem^.Next := tmpElem;
   CurElem := tmpElem;
end;
{***************************}
procedure ChangePos(var firstElem,secondElem:pEl);
var
   tmpElem:pEl;
begin
   New(tmpElem);

   tmpElem^.Prev := secondElem^.Prev;
   tmpElem^.Next := secondElem^.Next;

   secondElem^.Prev := firstElem^.Prev;
   secondElem^.Next := firstElem^.Next;

   firstElem^.Prev := tmpElem^.Prev;
   firstElem^.Next := tmpElem^.Next;
end;
{***************************}
begin
   clrscr;
   Randomize;
   New(firstElem);
   New(curElem);
   firstElem^.Next := nil;
   firstElem^.Prev := nil;
   firstElem^.Data := random(100);;

   curElem := firstElem;
   for i:=1 to MaxNum do
   begin
      AddElem(curElem,random(100));
      if i = 1 then firstElem^.Next := curElem;
      if i = 13 then
      begin
         New(PY);
         PY^.Prev := curElem^.Prev;
         PY^.Next := curElem^.Next;
         PY^.Data := curElem^.Data;
      end;
      if i = 20 then
      begin
         New(PX);
         PX^.Prev := curElem^.Prev;
         PX^.Next := curElem^.Next;
         PX^.Data := curElem^.Data;
      end;
   end;

   ChangePos(PX,PY);
   assign(f,'double~.txt');
   rewrite(f);
   writeln('FirstElement = ',firstElem^.Data);
   curElem := firstElem;
   while (curElem <> nil) do
   begin
       writeln(f, curElem^.Data);
       curElem := curElem^.Next;
   end;
   close(f);
   writeln('Press Enter');
   readln;
end.