in one shell : ledit bin/why3shell file.xml

in another shell : tail -f why3shell.out

The sum of two positive (resp. negative) FP numbers should always be
positive (resp. negative), even when the result is zero.

This commit also cleans a bit some other parts of the specification of FP
operations.

We need to stop the transformation intro_premises to introduce variables
past a label. This allows us to keep variables in the goal (for counterex
generation) and be able to retrieve them as counterexamples.

* transform/intro_vc_vars_counterexmp.ml:
  changed vc_term_info so that it is not mutable anymore
  (do_intro): Removing the passing records to the do_intros calls which
may prevent us from seeing last vc_model
  (do_intro_vc_vars): adding a reference to keep the location of the vc

* transform/introduction.ml
  (intros): When encountering stop_intro label, the function should
stop introducing.

Add the possibility to keep currently unavailable theories in a session
during update. So the proofs are not lost. Currently not activated in
why3 command line tools.

  added in 9577acb9

When Why3 is run on a file where some theories have been suppressed, it
will delete the corresponding theories from the session file.  We now
add an option keep_unmatched_theories to Session.update_session, which
keeps all theories. In this commit, this option is always disabled.

This is useful for SPARK, which sometimes only generates part of the
Why3 file for efficiency reasons, but doesn't want the session file to
be damaged because of that.

* session.ml
(import_theory)
(import_goal)
(import_proof_attempt)
(import_transf): new functions to copy a session tree from an old
  session file
(merge_file): keep old theories when keep_unmatched_theories is true
* session_scheduler.ml
(update_session): pass keep_unmatched_theories
* why3session_lib.ml
(read_update_session): pass keep_unmatched_theories