Rectangle Forcing Chains
Follows every branch of a choice and keeps what they all agree on.
All candidates have been entered in the empty cells.
At least one candidate in A is true. B marks the conclusion shared by every branch. On a solid link, one false end makes the other end true. On a dashed link, one true end makes the other end false. Apply these rules in link order in each branch. All 4 branches make B true. B is true for every possible A.