code: protected void registerNamespace(
  		SqlValidatorScope usingScope,
  		String alias,
  		SqlValidatorNamespace ns,
  		boolean forceNullable) {
  		namespaces.put(ns.getNode(), ns);
  		if (usingScope != null) {
  			usingScope.addChild(ns, alias, forceNullable);
  		}
  	}
docs: Registers a new namespace, and adds it as a child of its parent scope.
  Derived class can override this method to tinker with namespaces as they
  are created.
  
  @param usingScope    Parent scope (which will want to look for things in
  this namespace)
  @param alias         Alias by which parent will refer to this namespace
  @param ns            Namespace
  @param forceNullable Whether to force the type of namespace to be nullable