Recursive stack-based version of Back-chaining using Propositional Logic could be modified to handle... variables (using unification) negation loops (i.e. fail if sub-goal is repeated) Backchain(KB,query) stack {query} // initialize return BC(KB,stack)   BC(KB,stack) if stack empty, return True goal  stack.pop() if goalKB, return BC(KB,stack) // a known fact for each rule a1..angoal in KB: stack.push(a1..an) result  BC(KB,stack) if result=True, return True remove a1..an from stack return False

Hard Satisfiability Problems experiments with randomly generated Sat problems (clauses) "computational cliff" at around 4.3 clauses per symbol