1. A graph walking system, comprising: a binding system for binding a graph observer with a graph, for binding node patterns to node observers to generate at least one node pairing, and for binding the graph observer to at least one node pattern-node observer pairing; graph walking logic for systematically walking through nodes within the directed non-cyclic graph; a pattern testing system for determining if an encountered node matches one of the node patterns; an event manager for generating an encountered event when one of the node observers is bound to a matching node pattern; and a pruning system that can deactivate the graph observer with respect to sub-nodes of the encountered node if a bound node observer determines that there is no interest in the sub-nodes.