Split, combine ok but where's extract?

[David Vaughan]

Xavier, Alex and all.

The principal difficulty with the split/combine code I have suggested 
for extract/merge is that key order is not preserved in relation to the 
data when you get the keys. If you wish to re-merge after an extract 
then you must know what is the correct key order (e.g. a simple sort).

Oh, and I also reversed the data and keys in the second code block 
(first two lines) compared with the extract. Mechanical error.

regards
David