Forcing Y-Chain
A Forcing Y-Chain, commonly called an XY-Chain, uses bivalue cells as exactly-one nodes and connects them through weak inferences between equal digits in peer cells. The digit may change inside each bivalue cell; the link between cells is still essential.
What is a Forcing Y-Chain?
In a bivalue cell {A,B}, exactly one candidate is true. To move to another bivalue cell, the outgoing digit in the first cell and the same digit in the next cell must see each other, so they cannot both be true. Merely sharing a digit label without peer visibility does not connect the chain.
How to spot it
Trace bivalue cells so that each consecutive pair shares one candidate digit and the two occurrences of that digit see each other. Record the alternating exactly-one cell nodes and not-both intercell links. Then apply a valid open-endpoint or contradiction conclusion.
The logic
Inside a bivalue cell, false on one candidate makes the other true. Between peer cells, true on a shared digit makes the other occurrence false. A true candidate makes every weakly linked candidate false. A false candidate makes the other side of a strong inference true. False on one side of a weak inference alone proves nothing. A common-peer elimination needs endpoints proven to have an inclusive OR relation; a starting-candidate elimination needs a contradiction under that assumption.