Frauchiger & Renner / newby question
I am trying to understand Frauchiger & Renner's (2018) paper but it's been thirty years since I studies quantum physics at college.
There's one step I just can't get.
Fbar gets tails, and Wbar gets the result OK. Why?
Wbar is measuring the state of Fbar's lab against a basis that is the subtraction: (wave function for heads)  (wave function for tails).
So if Fbar gets tails, I expected Wbar to get fail.
Probably I don't understand what it means to use a basis that subtracts one wave function from another.
I am sure this must be a dumb question, but can anyone help me?
Reference
"Quantum theory cannot consistently describe the use of itself"
Daniela Frauchiger & Renato Renner
DOI 10.1038/s41467018057398
Nature Communications
