http://www.win.tue.nl/~hemerik/2IP15/Doc/D_5/Formula_Trees_RE.htm: 

type
  TREKind = (reEps, reElt, reDot, reStick, reStar);

  PRE = ^TRE;
  TRE =
  record
    case FKind: TREKind of
      reEps:
        ();
      reElt:
        (FName: Char);
      reDot, reStick:
        (FLeft, FRight: PRE);
      reStar:
        (FArg: PRE)
  end;