use ReducedFact to link original and WhatIf-reduced Facts
use ReducedFact to link original and WhatIf-reduced Facts

remove SMALL_TO_LARGE Comparator

refactor impliciations, byPopularity, isDepthAvailable, hasIllegalEmptyFact, claimsToExplore, and filteredReducedFacts

fiveham/Sudoku_Solver