where \([\Psi_1]\) and \([\Psi_2]\) are projectors onto the states \(\ket{\Psi_1}\) and \(\ket{\Psi_2}\) defined in \eqref{eqn20}, and the family \eqref{eqn21} should be interpreted as a set of four mutually exclusive histories obtained by choosing at \(t_1\) one of the projectors inside the first pair of curly brackets, and at \(t_2\) one inside the second pair. The corresponding chain kets, subsection 4.2, are zero except for the single history \([\Psi_0]\odot [\Psi_1] \odot [\Psi_2]\), which is assigned probability 1 by \eqref{eqn17}.