snappymail/vendors/knockout/src/binding/defaultBindings/let.js

ko.bindingHandlers['let'] = {
    'init': function(element, valueAccessor, allBindings, viewModel, bindingContext) {
        // Make a modified binding context, with extra properties, and apply it to descendant elements
        var innerContext = bindingContext['extend'](valueAccessor);
        ko.applyBindingsToDescendants(innerContext, element);

        return { 'controlsDescendantBindings': true };
    }
};
ko.virtualElements.allowedBindings['let'] = true;