"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.