// The background task should never have an event emitted for it.
			e.Errorf("found invalid task ID for task of type %s", t.Type)
		}
		if t.Parent == trace.BackgroundTask {
			// It's not possible for a task to be a subtask of the background task.
			e.Errorf("found background task as the parent for task of type %s", t.Type)
		}
		// N.B. Don't check the task type. Empty string is a valid task type.
		v.tasks[t.ID] = t.Type

type substitutions []AST

// add adds a new substitution candidate.
func (subs *substitutions) add(a AST) {
	*subs = append(*subs, a)
}

// subAST maps standard substitution codes to the corresponding AST.
var subAST = map[byte]AST{
	't': &Name{Name: "std"},
	'a': &Qualified{Scope: &Name{Name: "std"}, Name: &Name{Name: "allocator"}},

            // the pool core size will be incremented before submitting
            // all the tasks, then the thread will block waiting for all
            // those subtasks to finish.
            // This ensures the number of running workers is no more than
            // the defined parallism, while making sure the pool will not
            // be exhausted
            //

   *       the interrupt will be restored to the thread after it completes so that its {@code
   *       delegate} Executor may process the interrupt.
   *   <li>subtasks are run with the thread uninterrupted and interrupts received during execution
   *       of a task are ignored.
   * </ol>
   *

   *       the interrupt will be restored to the thread after it completes so that its {@code
   *       delegate} Executor may process the interrupt.
   *   <li>subtasks are run with the thread uninterrupted and interrupts received during execution
   *       of a task are ignored.
   * </ol>
   *

	Host xnet.Host // Remote host on which the rpc call was initiated
	Err  error     // Error returned by the remote peer for an rpc call
}

// A NotificationGroup is a collection of goroutines working on subtasks that are part of
// the same overall task.
//
// A zero NotificationGroup is valid and does not cancel on error.
type NotificationGroup struct {
	workers    *workers.Workers
	errs       []NotificationPeerErr