[wp/doc] Document induction tactic

src/plugins/wp/doc/manual/wp_plugin.tex

@@ -444,6 +444,18 @@ n|k, n|k', & (k/n).a = (k'/n).b &\Longleftrightarrow& k.a = k'.b
 \end{array}
 \]
 
+\paragraph{Induction} Start a proof by integer induction \\
+The user select any integer expression $e$ in the proof and a base value $b$ (which defaults to
+0). The tactic generates a proof by strong (integer) induction on $e$, that is, the base case
+$e = b$ and then the cases $e < b$ and $b < e$. Formally:
+
+\[\TACTIC{\Delta\models\,G}{%
+\begin{array}[t]{llll}
+\Delta, & G[e \leftarrow n], & n = b & \models G[e \leftarrow n] \\
+\Delta, & G[e \leftarrow i], & b \leq i < n & \models G[e \leftarrow n] \\
+\Delta, & G[e \leftarrow i], & n < i \leq b & \models G[e \leftarrow n]
+\end{array}} \]
+
 \paragraph{Overflow} Integer Conversions \\
 This tactic rewrites machine integer conversions by identity,
 providing the converted value is in available range. The tactic applies on expression