Agreed; the difference between &quot;easy&quot; and &quot;hard&quot; is whether a square always exists with one choice, but the difference between levels of &quot;hard&quot; is how complex a reasoning rule you have to apply.  No human should be expected to apply a brute-force backtracking solver, but humans can be expected to apply highly complex pattern-matching rules, or even invent new methods and sets of puzzles solvable with those methods.