Different Ways of Adding Loop Detection

Here are 3 possibilities:

• When a node is selected from the agenda, do not process if its state has already been processed. Warning Do not add the state to the exclusion list until you are sure it has been processed.
• When a node is expanded: do not expand the same state twice.
• When a node is put on the agenda: when adding nodes to the agenda (putl/3), check each to make sure that there are no other nodes with the same state.

Below we explore the second of these solutions in detail.