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 - use the syntax of the driver for the own realization of a theory
 - add a *_def lemma for defined logics with syntax
 - add comments that inform which syntax is used for declared logic

(from challenge 1 of verifythis@fm2012 competition *)

the Set signature is moved to a separate Extset module,
instead of being included into Extmap.S. The module hierarchy
becomes simpler, we don't shadow  OCaml standard modules anymore,
and the bug #15270 is fixed, too.