Validate the tree only once at creation.

container/container.go

@@ -84,6 +84,9 @@ func New(t terminalapi.Terminal, opts ...Option) (*Container, error) {
 	if err := applyOptions(root, opts...); err != nil {
 		return nil, err
 	}
+	if err := validateOptions(root); err != nil {
+		return nil, err
+	}
 	return root, nil
 }
 

container/options.go

@@ -35,7 +35,11 @@ func applyOptions(c *Container, opts ...Option) error {
 			return err
 		}
 	}
+	return nil
+}
 
+// validateOptions validates options set in the container tree.
+func validateOptions(c *Container) error {
 	// ensure all the container identifiers are either empty or unique.
 	var errStr string
 	seenID := map[string]bool{}