Browse_node offers a uniform interface to traverse constructions from * TypedTree. * * Mutually recursive types from TypedTree are wrapped into different * constructors of the type node. * Then the fold function traverses one-level of sub-nodes. * * In the meantime, the most specific environment and location are threaded * (FIXME: should these two be managed separately?). * * Finally BrowseT module a node into a tree which structure mimics * the recursive structure of the TypedTree node. *