"Continue" provides a good way to "page" results in the UI - the "next" button contains all "continue" values, and the handler appends them to the original query. But, there is no simple generic way to "reverse continue". I propose that continuable queries add an extra output block continuerev - that include all parameters needed to get the "previous page" of results. That might mean including opposite "dir" parameter, or the fact that it is now going in reverse.
If the user chooses to reverse the continuation direction on the result set #5 by appending continuerev instead of continue block to the original query, they would get previous page (result set #4), but the meaning of continue and continuerev should not switch meaning (otherwise the <previous> and <next> UI buttons would switch their meaning, or the user would have to use an extra parameter to keep track of the last usage direction. Using the continuerev block again should return the result set #3.