ast: print unknown node type

ast/walk.go

@@ -1,5 +1,7 @@
 package ast
 
+import "fmt"
+
 // Walk traverses an AST in depth-first order: It starts by calling fn(node);
 // node must not be nil.  If f returns true, Walk invokes f recursively for
 // each of the non-nil children of node, followed by a call of f(nil).
@@ -30,6 +32,8 @@ func Walk(node Node, fn func(Node) bool) {
 		for _, l := range n.List.Items {
 			Walk(l, fn)
 		}
+	default:
+		fmt.Printf(" unknown type: %T\n", n)
 	}
 
 	fn(nil)