code: function expose(){
  
      // TODO test this
      // if(!isDefined(defaultValue)){
      //  throw new Error("model.addPublicProperty() is being " +
      //    "invoked with an undefined default value. Default values for exposed properties " +
      //    "must be defined, to guarantee predictable behavior. For exposed properties that " +
      //    "are optional and should have the semantics of an undefined value, " +
      //    "use null as the default value.");
      //}
  
      // TODO test this
      if(!lastPropertyAdded){
        throw Error("Expose() was called without first adding a property.");
      }
  
      var propertyName = lastPropertyAdded;
  
      if(!exposedProperties){
        exposedProperties = [];
      }
      exposedProperties.push(propertyName);
  
      // Destroy the previous reactive function that was listening for changes
      // in all exposed properties except the newly added one.
      // TODO think about how this might be done only once, at the same time isFinalized is set.
      if(configurationReactiveFunction){
        configurationReactiveFunction.destroy();
      }
  
      // Set up the new reactive function that will listen for changes
      // in all exposed properties including the newly added one.
      var inputPropertyNames = exposedProperties;
  
      //console.log(inputPropertyNames);
      configurationReactiveFunction = ReactiveFunction({
        inputs: inputPropertyNames.map(getProperty),
        output: configurationProperty,
        callback: function (){
          var configuration = {};
          inputPropertyNames.forEach(function (propertyName){
            var property = getProperty(propertyName);
  
            // Omit default values from the returned configuration object.
            if(property() !== property.default()){
              configuration[propertyName] = property();
            }
          });
          return configuration;
        }
      });
  
      // Support method chaining.
      return model;
    }
docs: Exposes the last added property to the configuration.