Python: Implement ContentSet

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowPublic.qll

@@ -643,3 +643,20 @@ class AttributeContent extends TAttributeContent, Content {
 
   override string toString() { result = "Attribute " + attr }
 }
+
+/**
+ * An entity that represents a set of `Content`s.
+ *
+ * The set may be interpreted differently depending on whether it is
+ * stored into (`getAStoreContent`) or read from (`getAReadContent`).
+ */
+class ContentSet instanceof Content {
+  /** Gets a content that may be stored into when storing into this set. */
+  Content getAStoreContent() { result = this }
+
+  /** Gets a content that may be read from when reading from this set. */
+  Content getAReadContent() { result = this }
+
+  /** Gets a textual representation of this content set. */
+  string toString() { result = super.toString() }
+}