* temporary fix for eliminate_implications

* Change from << to <==

* Fixed implies operator in SAT_plan

* changed Expr name to counter

* Fixed a typo in logic.py

* Added translate_to_SAT()

* extract solution from model

* added test cases

* removed debug code

Add InfixOps, refactor expo and Expr

- Fix relative imports in base of is a package or not
- Fix not working notebooks
- Add notebooks to packages sources

Used tool 2to3 to finish the porting to Python 3 plus some fixes by
hand.

These functions, which I had to implement in Python 2.2, are now
available in standard Python.

use “raise NotImplementedError” for abstract methods

Agent.__init__ now takes a program parameter. A few more nonlocal-state-variable problems found and fixed. One agent program rewritten as a callable instance since it uses both state variables and abstract methods.

Moved agent-program definition back into the constructors to get rid of all the copying between parameters, instance vars, and nonlocals. Fixed nonlocal state-var bug in ReflexAgentWithState.