Techniques › 8 of 9
Edge Logic & 2-Line Lookahead
Level 4: Unfair
Test a cell and follow its immediate effect on the crossing lines; if a neighboring line breaks, the cell is decided.
Suppose the highlighted cell in row 2 were filled. That would force a neighboring cell in the same row, and row 3 would then be left with no valid arrangement of its clues.
Because filling it breaks a line, it must be crossed out. This is a one-step lookahead, not a guess: you only follow forced consequences.