add a tree structure fixer

Parser/spec/Helpers.jadd

+aspect Helpers {
+
+    public void ASTNode.checkTreeStructure() {
+        for (ASTNode child : astChildren()) {
+            if (child.getParent() != this) {
+                throw new RuntimeException();
+            }
+            child.checkTreeStructure();
+        }
+    }
+
+    public void ASTNode.fixTreeStructure() {
+        for (ASTNode child : astChildren()) {
+            if (child.getParent() != this) {
+                // System.err.println("fixed parent of " + child);
+                child.parent = this;
+            }
+            child.fixTreeStructure();
+        }
+    }
+
+}