* simple test for conjunction duplicates
* fixing an incorrect order of insertion and duplication visitors in the
tracing engine
* other smallish fixes and comments
properly injecting the useless traced propagations into the chain of
inferences
test for duplication of existential inferences
implemented hybrid contexts to avoid duplicate inference during traces,
added two simple unit tests (one for conjunction, the other for
existential). also implemented generic multimap filtering and union
operations to filter backward links in hybrid contexts.