problem with an algorithm

[ErwinM <funnyacc@gmx.net>]

Hallo
this is my first threat in this newsgroup and I'm a newbe about caml so I 
need some usefull help with a problem I have.

I have two lists , f.e.: List1 = [1;2;3;4] and List2 = [5;4]. And I will 
write a program that can take subtraction in the lists.
example: [1;2;3;4] - [5;4] = [1;1;8;0]
         ( 1234 - 54 = 1180 )

I have already a program that subtract 2 lists but the other way round.
example: [1;2;3;4] - [5;4] = [6;7;2;4]
   ( 4321 - 45 = 4276 )
this code do this:

let carry x = if x < 0 then 1 else 0
and sub   x = if x < 0 then 10 + x else x;;

let rec Subtraktion = function

  ([]   , []   , c)-> if c = 0 then [] else [c]
| (a::ta, []   , c)-> sub (a-c)  ::Subtraktion(ta, [], carry(a-c))
| ([]   , b::tb, c)-> sub (b-c)  ::Subtraktion([], tb, carry(b-c))
| (a::ta, b::tb, c)-> sub (a-b-c)::Subtraktion(ta, tb, carry(a-b-c));;

How I wrote, I need a program that substract the real number [1;2;3;4] = 
1234 and [5;4] = 54

But I can't use functions that just turn over the two lists! If I can use 
this function so it will me very easy.
But I can't and so I spent hours with no idea and I need some help.

--
Sorry for my english ;)
So Long, Erwin