An ExitPortal is just like an Exitable, except that you can go through it as well as exit it. For example:
squareCave : DarkRoom 'Square Cave' 'the square cave'
"This capacious cave is unnaturally square, suggesting that it has been
artificially hewn out of the rock, an impression further enhanced by
the carefully-constructed ashlar archway to the west. "
west = mainCave
+ ExitPortal -> mainCave 'ashlar arch/archway' 'archway'
"The archway is beautifully constructed from dressed stones. "
Note that ExitPortal is not a travel connector; -> mainCave makes mainCave its connector property, not its masterObject property. For the distinction, see further on the Enterable class. The template used here is the Exitable template.