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.