Datatyping desiderata - take 3

Some small changes this time:

- noted item 7 seems to duplicate 5/6 (not 4/5)

- added a little more explanatory text in the section on idioms

- presented all idioms using triples

- re-labelled the idioms A, B, C and D to reduce confusion about their 
being specific to proposals (but still acknowledging their origin in 
proposals).

#g


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