$\forall$$P_{1}$, $P_{2}$, $Q_{1}$, $Q_{2}$:$\mathbb{P}$. \{$P_{1}$ $\Leftarrow\!$ $P_{2}$\} $\Rightarrow$ \{$Q_{1}$ $\Rightarrow$ $Q_{2}$\} $\Rightarrow$ \{($P_{1}$ $\Rightarrow$ $Q_{1}$) $\Rightarrow$ $P_{2}$ $\Rightarrow$ $Q_{2}$\}