Fixed error in Chart parses method, where it would
incorrectly return some parses that did not start at the beginning
of the input (and therefore did not span the whole input).

Corrected doctests for standardize_apart.

Preliminary doctest examples for fol_bc_ask.

Corrected error in the 'literals' function.

In class FolKB, added doctest examples; added definition
of method ask_generator.

Added functions pretty, pretty_dict, pretty_set, pp;
redefined ppdict, ppset, using pretty_dict and pretty_set.

Cleaned up code, simplifying statements and comments.

fol_bc_ask MIGHT be working now, in the form of returning a list of all 
answers instead of yielding them one at a time.  Needs more testing.

literals, is_definite_clause.

Partially implemented FolKB class and function fol_bc_ask.

Fixed errors in pl_resolution, standardize_apart.

Fixed errors in doctest cases for dpll_satisfiable, unify, extend,
and pl_resolve.

Added functions ppsubst, ppdict, and ppset, and used to simplify doctest cases.