"slicing" records

Hello

I have a table with objects' descriptions:

   id    | length
---------+--------
 object1 |     40
 object2 |     66
 object3 |     12
 object4 |    107
 object5 |    220

But I need to export data to a legacy system that doesn't handle lengths
greater than 50 (don't ask me why...). Instead, it expects the data in this
format:

   id    | length | fragment | offst
---------+--------+----------+-------
 object1 |     40 | whole    |     0
 object2 |     50 | start    |     0
 object2 |     16 | end      |    50
 object3 |     12 | whole    |     0
 object4 |     50 | start    |     0
 object4 |     50 | middle   |    50
 object4 |      7 | end      |   100
 object5 |     50 | start    |     0
 object5 |     50 | middle   |    50
 object5 |     50 | middle   |   100
 object5 |     50 | middle   |   150
 object5 |     20 | end      |   200

So when length becomes greater, it is break up in as many pieces as
necessary, each of max allowed length except the last one, in such a way
that the sum of partial lengths equals the original one.

Now I couldn't manage to get a query capable of doing this. If anybody has
an idea, I'll be very much appreciated.

TIA,
cl.