A collection of UtilityPropagator objects to execute while performing the trace. Propagator objects allow a subset of UtilityNetworkAttribute values to propagate through a network while executing a trace. For example, in phase propagation, open devices along the network will restrict some phases from continuing along the trace. Propagators only apply to subnetwork-based traces. (UtilityTraceType.Upstream, UtilityTraceType.Downstream, and so on).