The Abbr function is used to signal to various simplification
tactics that an abbreviation in the current goal should be eliminated
before simplification proceeds.  Each theorem created by Abbr is
removed from the tactic’s theorem-list argument, and causes a call to
Q.UNABBREV_TAC with that Abbr theorem’s argument.  Finally, the
simplification tactic continues, with the rest of the theorem-list as
its argument.  Thus,
   tac [..., Abbr`v`, ..., Abbr`u`, ...]
   Q.UNABBREV_TAC `v` THEN Q.UNABBREV_TAC `u` THEN
   tac [..., ..., ...]