Writing NAnt tasks with nested types

"I imagined a task that I could give a FileSet to, a regular expression to match, and a string to replace that match with. It's a sharp tool that you can easily break things with if you're not careful, but incredibly powerful if you are careful! So we set out to make our task where we'd like to have a FileSet. In your public instance properties, make a field that is the NAnt.Core.Types.FileSet, and add some getter/setter code." [Code samples included]