Returns a tree that will be the result of executing the indicated expression in the context of the indicated tree.
This also watches the tree for change events. Currently, FileTree is the only type of tree that supports change events. If a change event is raised, the expression is reevaluated to obtain a new result tree. Subsequent references to the watch() result will access the new tree.
This can be used to serve a virtual folder, reevaluating its definition whenever the containing real folder changes.
$ ori @serve @watch src, =site.ori/public