-First crack at indices for fact bases
-Added remove! method to index class
-Fixed empty ref bug in index function
-`index` function renamed to `index-by`
-Re-jigged order of arguments to `index-by`
-Commented out `delete!` (not sure this method jives with the index approach; thinking about just storing fact literals, partial or not, for deletion)
-Fixed bugs in the index-by method of fact-base