Sismic

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- (Changed) ``Event``, ``MacroStep``, ``MicroStep``, ``StateMixin``, ``Transition``, ``Statechart`` and
``Interpreter``'s ``__repr__`` returns a valid Python expression.
- (Changed) The context returned by a ``PythonEvaluator`` (and thus by the default ``Interpreter``) exhibits
nested variables (the ones that are not defined in the preamble of a statechart). Those variables are prefixed by
the name of the state in which they are declared, to avoid name clashing.
- (Changed) Context variables are sorted in exceptions'``.__str__`` methods.

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- (Added) ``sismic-behave`` CLI now accepts a ``--steps`` parameter, which is a list of file paths containing the steps
implementation.
- (Added) ``sismic-behave`` CLI now accepts a ``--show-steps`` parameter, which list the steps (equivalent to
Behave's overriden ``--steps`` parameter).
- (Added) ``sismic-behave`` now returns an appropriate exit code.
- (Changed) Reorganisation of ``docs/examples``.
- (Fixed) Coverage data for ``sismic-behave`` takes the initialization step into account (regression
introduced in 0.21.0).

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- (Added) ``sent`` and ``received`` are also available in preconditions and postconditions.

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- (Added) ``model.Event`` is now correctly pickled, meaning that Sismic can be used in a multiprocessing environment.

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- (Added) A *event {event_name} should not be fired* steps for BDD.
- (Added) Both ``MicroStep`` and ``MacroStep`` have a list ``sent_events`` of events that were sent during the step.
- (Added) Property statecharts receive a ``event sent`` event when an event is sent by the statechart under test.
- (Changed) Events fired from within the statechart are now collected and sent at the end of the current micro step,
instead of being immediately sent.
- (Changed) Invariants and sequential contracts are now evaluated ordered by their state's depth

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- (Added) Support for sequential conditions in contracts (see documentation for more information).
- (Added) Python code evaluator: *after* and *idle* are now available in postconditions and invariants.
- (Added) Python code evaluator: *received* and *sent* are available in invariants.
- (Added) An ``Evaluator`` has now a ``on_step_starts`` method which is called at the beginning of each step, with
the current event (if any) being processed.
- (Added) ``Interpreter.raise_event`` to send events from within the statechart.
- (Added) A ``copy_from_statechart`` method for a ``Statechart`` instance that allows to copy (part of) a statechart
into a state.
- (Added) Microwave controller example (see *docs/examples/microwave.[yaml|py]*).
- (Changed) Events sent by a code evaluator are now returned by the ``execute_*`` methods instead of being
automatically added to the interpreter's queue.
- (Changed) Moved ``run_in_background`` and ``log_trace`` from ``sismic.interpreter`` to the newly added
``sismic.interpreter.helpers``.
- (Changed) Internal API changes: rename ``self.__x`` to ``self._x`` to avoid (mostly) useless name mangling.