\bigvee_{n=0}^N T^{-n}Q = \left \{Q_{i_0} \cap T^{-1}Q_{i_1} \cap \cdots \cap T^{-N}Q_{i_N} \text{ where } i_\ell = 1,\ldots,k ,\ \ell=0,\ldots,N,\ \mu \left (Q_{i_0} \cap T^{-1}Q_{i_1} \cap \cdots \cap T^{-N}Q_{i_N} \right )>0 \right \} .
which plays crucial role in the construction of the measure-theoretic entropy of a dynamical system.