2. 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

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