User defined type conversion

["Vinícius Franchini" <viniciusnf@gmail.com>]

Hi Folks,
 I have a problem with a type conversion. Hereby is what I'm trying to do:

type AB is tagged
  record
    A : Unsigned_16;
    B : Unsigned_16;
  end record;

type abcde is tagged
  record
    a: Unsigned_8;
    b: Unsigned_8;
    c: Unsigned_8;
    d: Unsigned_8;
  end record;

X : AB;
Y : abcd;

X := Y;

I'd like to do a direct conversion without creating a method for it.