void (Namespace additionalNamespace) { if (additionalNamespaces == null) { additionalNamespaces = new ArrayList<>(); } else { for (Namespace ns : additionalNamespaces) { if (ns == additionalNamespace) { return; } } } // Verify the new namespace prefix doesn't collide with another // declared namespace, an attribute prefix, or this element's prefix final String reason = Verifier.checkNamespaceCollision(additionalNamespace, this); if (reason != null) { throw new IllegalAddException(this, additionalNamespace, reason); } additionalNamespaces.add(additionalNamespace); }