Maintaining EXCL

The list of states already expanded (called EXCL for ``exclude'') is maintained as an extra argument to agenda_search and initialised to [ ]. We then update EXCL as follows.

The single last clause of agenda_search_1 is replaced with a pair of clauses:

• The first handles the case where the state S of the current node N has already been visited. In such a case, don't expand (and don't backtrack to the next clause, hence the cut).
• The second expands the node, and puts its state S on the exclusion list.