code: public final R visit(Iterable<? extends DocTree> nodes, P p) {
          R r = null;
          if (nodes != null) {
              for (DocTree node : nodes)
                  r = visit(node, p);
          }
          return r;
      }
docs: Invokes the appropriate visit method on each of a sequence of nodes.
  @param nodes the nodes on which to dispatch
  @param p a parameter value to be passed to each appropriate visit method
  @return the value return from the last of the visit methods, or null
  if none were called.