  The extended Wigner's friend problem deals with two Observers each measuring
a sealed laboratory in which a friend is making a quantum measurement. We
investigate this problem by relying on the basic rules of quantum mechanics as
exposed by Feynman in the well-known "Feynman Lectures on Physics". Although
recent discussions have suggested that the extended Wigner's friend problem
cannot consistently be described by quantum theory, we show here that a
straightforward application of these standard rules results in a non-ambiguous
and consistent account of the measurement outcomes for all agents involved.
