Datatyping desiderata, take 4

My apologies for being so late with this...

Together with the changes in "take 3", I think that this addresses the 
issues raised previously.

- Added idioms E and F, based on suggestions by Brian McBride.

- Replaced idiom C with reference to idiom B, since they had the same form.

- Give some indication of where the various idioms have been used.

- Some editorial changes and attempted clarifications.

#g



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