--- /dev/null
+\section{Implementation}
+\subsection{Screen encoding}
+When parsed the sokoban screen is stripped of all walls and unreachable empty
+spaces are removed.
+
+Let $T=\{free,box,target,agent,targetagent,targetbox\}$ be the set of
+possible states of a tile. Tiles are numbered and thus a sokoban screen is the
+set $F$ containing a $x_i\in T$ for every tile. To encode the state we
+introduce an encoding function that encodes a state in three boolean variables:
+$$encode(t)=\begin{cases}
+ 000 & \text{if }t=wall\\
+ 001 & \text{if }t=free\\
+ 010 & \text{if }t=box\\
+ 011 & \text{if }t=target\\
+ 100 & \text{if }t=targetbox\\
+ 101 & \text{if }t=agent\\
+ 110 & \text{if }t=agentbox
+\end{cases}$$
+
+This means that the encoding of a screen takes $3*|F|$ variables.
+
+\subsection{Transition encoding}
+We introduce a variable denoting the intended direction of movement $m \in
+\{\text{up}, \text{down}, \text{left}, \text{right}\}$.
+%
+%Let\\
+%$\delta_{x}(x,m) =
+% \begin{cases}
+% (x+1) & \quad \text{if } m = left\\
+% (x-1) & \quad \text{if } m = right\\
+% x & \quad \text{otherwise}
+% \end{cases}$\quad
+%$\delta'_{x}(x,m) =
+% \begin{cases}
+% (x-1) & \quad \text{if } m = left\\
+% (x+1) & \quad \text{if } m = right\\
+% x & \quad \text{otherwise}
+% \end{cases}$\\
+%$\delta_{y}(y,m) =
+% \begin{cases}
+% (y+1) & \quad \text{if } m = up\\
+% (y-1) & \quad \text{if } m = down\\
+% y & \quad \text{otherwise}
+% \end{cases}$\quad
+%$\delta'_{y}(y,m) =
+% \begin{cases}
+% (y-1) & \quad \text{if } m = up\\
+% (y+1) & \quad \text{if } m = down\\
+% y & \quad \text{otherwise}
+% \end{cases}$\\
+% $\gamma_{x}(x,m) =
+% \begin{cases}
+% (x+2) & \quad \text{if } m = left\\
+% (x-2) & \quad \text{if } m = right\\
+% x & \quad \text{otherwise}
+% \end{cases}$\quad
+% $\gamma_{y}(y,m) =
+% \begin{cases}
+% (y+2) & \quad \text{if } m = up\\
+% (y-2) & \quad \text{if } m = down\\
+% y & \quad \text{otherwise}
+% \end{cases}$
+%
+%% " x" <+ x <+ "_" <+ y <+ " = BoxOnTarget & (x" <+ (checkX p x) <+ "_" <+ (checkY p x (y+1)) <+ " = Agent | x"
+%% <+ (checkX p x) <+ "_" <+ (checkY p x (y+1)) <+ " = AgentOnTarget) & (x" <+ (checkX p x) <+ "_" <+ (checkY p x (y-1)) <+ " = Free | x" <+ (checkX p x) <+ "_" <+ (checkY p x (y-1)) <+ " = Target) & move = Left: AgentOnTarget;",
+%% " x" <+ x <+ "_" <+ y <+ " = BoxOnTarget & (x" <+ (checkX p (x+1)) <+ "_" <+ (checkY p (x+1) y) <+ " = Agent | x"
+%% <+ (checkX p (x+1)) <+ "_" <+ (checkY p (x+1) y) <+ " = AgentOnTarget) & (x" <+ (checkX p (x-1)) <+ "_" <+ (checkY p (x-1) y) <+ " = Free | x" <+ (checkX p (x-1)) <+ "_"
+%% <+ (checkY p (x-1) y) <+ " = Target) & move = Up: AgentOnTarget;",
+%% " x" <+ x <+ "_" <+ y <+ " = BoxOnTarget & (x" <+ (checkX p x) <+ "_" <+ (checkY p x (y-1)) <+ " = Agent | x"
+%% <+ (checkX p x) <+ "_" <+ (checkY p x (y-1)) <+ " = AgentOnTarget) & (x" <+ (checkX p x) <+ "_" <+ (checkY p x (y+1)) <+ " = Free | x" <+ x <+ "_" <+ (checkY p x (y+1)) <+ " = Target) & move = Right : AgentOnTarget;",
+%% " x" <+ x <+ "_" <+ y <+ " = BoxOnTarget & (x" <+ (checkX p (x-1)) <+ "_" <+ (checkY p (x-1) y) <+ " = Agent | x"
+%% <+ (checkX p (x-1)) <+ "_" <+ (checkY p (x-1) y) <+ " = AgentOnTarget) & (x" <+ (checkX p (x+1)) <+ "_" <+ (checkY p (x+1) y) <+ " = Free | x" <+ (checkX p (x+1)) <+ "_"
+%% <+ (checkY p (x+1) y) <+ " = Target) & move = Down : AgentOnTarget;",
+%
+%
+%
+%We define the tile update function $next(x_{i,j}), x_{i,j} \in F, \forall i,j \text{ s.t.} x_{i,j} \neq \perp$ as\\
+%$
+%next(x_{i,j}) =
+% \begin{cases}
+% \# & \quad \text{if } x_{i,j} = \#\\
+% @ & \quad \text{if } x_{i,j} = \_ \wedge (x_{\delta_{x}(i,m),\delta_{y}(j,m)} = @ \vee x_{\delta_{x}(i,m),\delta_{y}(j,m)} = +) \wedge x_{\delta_{x}(i,m),\delta_{y}(j,m)} \neq \perp\\
+% @ & \quad \text{if } x_{i,j} = \$ \wedge (x_{\delta_{x}(i,m),\delta_{y}(j,m)} = @ \vee x_{\delta_{x}(i,m),\delta_{y}(j,m)} = +)\\
+% & \quad \wedge (x_{\delta'_{x}(i,m),\delta'_{y}(j,m)} = \_ \vee x_{\delta'_{x}(i,m),\delta'_{y}(j,m)} = .)\\
+% & \quad \wedge x_{\delta_{x}(i,m),\delta_{y}(j,m)} \neq \perp \wedge x_{\delta'_{x}(i,m),\delta'_{y}(j,m)} \neq \perp\\
+% \$ & \quad \text{if } x_{i,j} = \_ \wedge (x_{\delta_{x}(i,m),\delta_{y}(j,m)} = \$ \vee x_{\delta_{x}(i,m),\delta_{y}(j,m)} = *)\\ & \quad \wedge (x_{\gamma_{x}(i,m),\gamma_{y}(j,m)} = @ \vee x_{\gamma_{x}(i,m),\gamma_{y}(j,m)} = +) \wedge x_{\delta_{x}(i,m),\delta_{y}(j,m)} \neq \perp \wedge x_{\gamma_{x}(i,m),\gamma_{y}(j,m)} \neq \perp\\
+% \_ & \quad \text{if } x_{i,j} = @ \wedge (x_{\delta_{x}(i,m),\delta_{y}(j,m)} = \_ \vee x_{\delta_{x}(i,m),\delta_{y}(j,m)} = .)\\
+% & \quad \vee ((x_{\delta_{x}(i,m),\delta_{y}(j,m)} = \$ \vee x_{\delta_{x}(i,m),\delta_{y}(j,m)} = *) \wedge \\
+% & \quad (x_{\gamma_{x}(i,m),\gamma_{y}(j,m)} = \_ \vee x_{\gamma_{x}(i,m),\gamma_{y}(j,m)} = .)) \\
+% & \quad x_{\delta_{x}(i,m),\delta_{y}(j,m)} \neq \perp \wedge x_{\gamma_{x}(i,m),\gamma_{y}(j,m)} \neq \perp\\
+% + & \quad x_{i,j} = . \wedge (x_{\delta_{x}(i,m),\delta_{y}(j,m)} = @ \vee x_{\delta_{x}(i,m),\delta_{y}(j,m)} = +) \wedge x_{\delta_{x}(i,m),\delta_{y}(j,m)} \neq \perp\\
+% + & \quad x_{i,j} = * \wedge (x_{\delta_{x}(i,m),\delta_{y}(j,m)} = @ \vee x_{\delta_{x}(i,m),\delta_{y}(j,m)} = +)\\
+% & \quad \wedge (x_{\delta'_{x}(i,m),\delta'_{y}(j,m)} = \_ \vee x_{\delta'_{x}(i,m),\delta'_{y}(j,m)} = .)\\
+% & \quad \wedge x_{\delta_{x}(i,m),\delta_{y}(j,m)} \neq \perp \wedge x_{\delta'_{x}(i,m),\delta'_{y}(j,m)} \neq \perp\\
+% * & \quad \text{if } x_{i,j} = . \wedge (x_{\delta_{x}(i,m),\delta_{y}(j,m)} = \$ \vee x_{\delta_{x}(i,m),\delta_{y}(j,m)} = *)\\
+% & \quad \wedge (x_{\gamma_{x}(i,m),\gamma_{y}(j,m)} = @ \vee x_{\gamma_{x}(i,m),\gamma_{y}(j,m)} = +)\\
+% & \quad \wedge x_{\delta_{x}(i,m),\delta_{y}(j,m)} \neq \perp \wedge x_{\gamma_{x}(i,m),\gamma_{y}(j,m)} \neq \perp\\
+% . & \quad x_{i,j} = + \wedge (x_{\delta_{x}(i,m),\delta_{y}(j,m)} = \_ \vee x_{\delta_{x}(i,m),\delta_{y}(j,m)} = .) \wedge x_{\delta_{x}(i,m),\delta_{y}(j,m)} \neq \perp\\
+% x_{i,j} & \quad \text{otherwise}\\
+% \end{cases}
+%$
+%\subsection{Goal}
+%Let $G = \{z_{i,j} | z_{i,j} \in F, \forall i,j \text{ s.t.} z_{i,j} \in \{.,*\}\}$ be a subset of $F$.
+%In order to check a sokoban field for a possible solution, we introduce the following invariant:\\
+%$$\neg \bigwedge_{x \in G} (x = *)$$