more operations;

--- a/src/Pure/PIDE/document.scala	Fri Nov 01 18:19:32 2019 +0100
+++ b/src/Pure/PIDE/document.scala	Fri Nov 01 18:41:52 2019 +0100
@@ -408,6 +408,7 @@
       } yield cmd).toList
 
     def descendants(names: List[Node.Name]): List[Node.Name] = graph.all_succs(names)
+    def requirements(names: List[Node.Name]): List[Node.Name] = graph.all_preds(names).reverse
     def topological_order: List[Node.Name] = graph.topological_order
 
     override def toString: String = topological_order.mkString("Nodes(", ",", ")")