Does Backward chaining implicitly uses "Back Tracking" technique to get the previous value if no current value at the time of refering target property.Back Tracking solves N-Queens problem, i think Pega follows the same approach here too.
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Internally, Pega 7 Platform uses an inference engine and the declarative network to support declarative processing. Two properties A and B are linked in the network if the value of A depends on the value of B.
Inference engine processing operates invisibly in the moment between activity step executions. The step most recently executed may have changed a property value (and cause forward chaining, defined below), or may have updated the database (and cause Declare OnChange processing). The next step may need a property value that is not yet defined, which causes backward chaining to start.