Hi I added a "hint button" for indication of right columns and rows.
It's still unclear to me even with hint turned on. I don't understand why a line is considered correct even though the hint says so.
Speaking of hint button, I clicked hint button after it ... gives hint to turn it off and click it again to turn it on, and the hints are gone. I didn't change anything on the board at all.
You really should add tutorial levels at first showing game's rules from basic.