[Haskell-beginners] do-notation question

[blackcat at pro-ns.net]

 In The Yesod Book, I see the following code:

 defaultLayout contents = do
         PageContent title headTags bodyTags <- widgetToPageContent $ do
             addCassius ...

 How does this work?  Is bodyTags drawn from widgetToPageContent and 
 then used as the third argument to PageContent?

 tia,
 Lee