Suggested modification to Online-DFS-Agent (aima3e Figure 4.21)

[ctjoreilly]

To avoid cycles in the DFS traversal, modifying the following conditional from:

…
if s is not null then
  result[s, a] ← s'
  add s to front of unbacktracked[s']
…

to:

if s is not null and s′ ≠ result[s, a] then
  result[s, a] ← s'
  add s to front of unbacktracked[s']
…

without this additional check it is very easy to end up cycling between states, depending on the order of the actions that are tried from each state. Both the current aima-java and aima-python reference implementations contain this additional check.