+% \clearpage\r
+% \section*{NP-hardness}\r
+% \newcommand{\Pacman}{\Omega}\r
+% \newcommand{\sep}{\cdot}\r
+% \begin{example}[Graph 3-coloring]\label{example:3col}\r
+% Let $G=(V,E)$ be a graph, with $N \defeq |V|$.\r
+% We encode the problem of finding a 3-coloring of $G$ in the following problem of semi-$\sigma$-separation:\r
+% \[\begin{array}{cl}\r
+% \Uparrow & x \sep t_1^1\,t_1^2\,t_1^3 \sep t_2^1\,t_2^2\,t_2^3 \sep \cdots t_n^1\,t_n^2\,t_n^3 \\\r
+% \\\r
+% \Downarrow & x \sep \Pacman\,\Pacman\,\Pacman \sep t_2^1\,t_2^2\,t_2^3 \sep \cdots t_n^1\,t_n^2\,t_n^3 \\\r
+% \Downarrow & x \sep t_1^1\,t_1^2\,t_1^3 \sep \Pacman\,\Pacman\,\Pacman \sep \cdots t_n^1\,t_n^2\,t_n^3 \\\r
+% \vdots & \vdots \\\r
+% \Downarrow & x \sep t_1^1\,t_1^2\,t_1^3 \sep t_2^1\,t_2^2\,t_2^3 \sep \cdots \Pacman\,\Pacman\,\Pacman \\\r
+% \end{array}\]\r
+%\r
+% Where: $\dummy$ is (probably) a variable, $\bomb\defeq \lambda\_.\,\bot$, and the $a$'s are defined as follows:\r
+%\r
+% \begin{itemize}\r
+%\r
+% \item $\begin{array}{ll}\r
+% a_1^1 \defeq & \lambda\_. \, x \sep y\bomb\bomb \sep\bomb\ldots \\\r
+% a_1^2 \defeq & \lambda\_. \, x \sep \bomb y\bomb \sep\bomb\ldots \\\r
+% a_1^3 \defeq & \lambda\_. \, x \sep \bomb\bomb y \sep\bomb\ldots \\\r
+% \end{array}$\r
+%\r
+% \item $a_2^1 \defeq \begin{cases}\r
+% \lambda\_.\, x \sep \bomb \dummy\dummy \sep y\bomb\bomb \cdot \bomb\ldots & \text{if } (v_1, v_2)\in E \\\r
+% \lambda\_.\, x \sep \dummy\dummy\dummy \sep y\bomb\bomb \cdot \bomb\ldots & \text{if } (v_1, v_2)\not\in E \\\r
+% \end{cases}$\r
+%\r
+% \item \ldots\r
+%\r
+% \end{itemize}\r
+%\r
+% \begin{definition}[Index notation]\r
+% Let $t = x \sep x_1^1 x_2^1 x_3^1 \sep x_1^2 x_2^2 x_3^2 \sep \ldots \sep x_1^m x_2^m x_3^m$. Then: \[t[\,_k^j] \defeq x_k^j.\]\r
+% \end{definition}\r
+%\r
+% Let $z_0$, $z_1$, $z_2$ be variables.\r
+%\r
+% Define:\r
+% \[a_k^j[\,_{k'}^{j'}]\defeq\begin{cases}\r
+% \bomb & \text{if } j<j' \\\r
+% \begin{cases}\r
+% \bomb & \text{if } k\neq k' \\\r
+% y & \text{if } k = k' \\\r
+% \end{cases} & \text{if } j = j' \\\r
+% \begin{cases}\r
+% \bomb & \text{if } k = k' \\\r
+% \dummy & \text{if } k \neq k' \\\r
+% \end{cases} & \text{if } (v_j,v_{j'}) \in E \\\r
+% \dummy & \text{if } (v_j,v_{j'}) \not\in E\r
+% \end{cases}\]\r
+%\r
+% Attenzione! Le $a$ vanno protette da lambda ($\lambda\_$)!\r
+%\r
+% % Dimensione del problema: circa $(3\times m^2)^2$.\r
+%\r
+% Intuitively, if $\sigma(x)$ ``uses'' $a_j^i$, then $\sigma$ colors $v_j$ with color $i$.\r
+%\r
+% \begin{lemma}[Extraction of the coloring]\r
+% Let $\sigma$ be a substitution which is a solution for the semi-separation problem. Then for example:\r
+%\r
+% \begin{itemize}\r
+% \item $\operatorname{color}(v_1) = 2$ iff\r
+% \[(x \sep \Pacman\,\bomb\,\Pacman \sep \bomb\,\bomb\,\bomb \,\sep \bomb\,\bomb\,\bomb \,\sep \cdots \sep \bomb\,\bomb\,\bomb)\,\sigma \to \bot\]\r
+% \item $\operatorname{color}(v_2) = 3$ iff:\r
+% \[(x \sep \dummy\,\dummy\,\dummy \sep \Pacman\,\Pacman\,\bomb \,\sep \bomb\,\bomb\,\bomb \,\sep \cdots \sep \bomb\,\bomb\,\bomb)\,\sigma \to \bot\]\r
+% \end{itemize}\r
+% \end{lemma}\r
+%\r
+% % Where $\Pacman \equiv \lambda\_.\,\Pacman$.\r
+%\r
+% \end{example}\r
+\r