added matplotlib post-processor for static drawings.
added error test case.
added error test case.
stabilize error dump.
Merge ../incoming into HEAD
`dump_rules` won't crash when rule failed to load dyna source.

Show failure context when an update handler crashes (work in progress)

when an initializer fails rules is still added to the program. Only some of the
emits which succeeded get scheduled (this isn't quite right).

added depth limit argument to trace (available via --limit=<INTEGER> at the
repl).
better cleanup when an initializer fails.
Stabilize output of `bag=` by sorting elements and properly handle null.

Uninitialized rules are kept around and appear as errors in the error dump,
until initialization succeeds.

Fixed sort order for true and false.
BUGFIX: properly clear error.

run initializers after clearing agenda, if new updates are queued run agenda
again.
new tests for error handling.
minor cleanup.
move command-line interface to separate file --> main.py

added a few test for "known bugs" regarding retracting BC rules.