\begin{module}[id=bbt-size]
\importmodule[balanced-binary-trees]{balanced-binary-trees}
\importmodule[\KWARCslides{dmath/en/cardinality}]{cardinality}
\begin{frame}
\frametitle{Size Lemma for Balanced Trees}
\begin{itemize}
\item
\begin{assertion}[id=size-lemma,type=lemma]
Let $G=\tup{V,E}$ be a \termref[cd=binary-trees]{balanced binary tree}
of \termref[cd=graph-depth,name=vertex-depth]{depth}$n>i$, then the set
$\defeq{\livar{V}i}{\setst{\inset{v}{V}}{\gdepth{v} = i}}$ of
\termref[cd=graphs-intro,name=node]{nodes} at
\termref[cd=graph-depth,name=vertex-depth]{depth} $i$ has
\termref[cd=cardinality,name=cardinality]{cardinality} $\power2i$.
\end{assertion}
\item
\begin{sproof}[id=size-lemma-pf,proofend=,for=size-lemma]{via induction over the depth $i$.}
\begin{spfcases}{We have to consider two cases}
\begin{spfcase}{$i=0$}
\begin{spfstep}[display=flow]
then $\livar{V}i=\set{\livar{v}r}$, where $\livar{v}r$ is the root, so
$\eq{\card{\livar{V}0},\card{\set{\livar{v}r}},1,\power20}$.
\end{spfstep}
\end{spfcase}
\begin{spfcase}{$i>0$}
\begin{spfstep}[display=flow]
then $\livar{V}{i-1}$ contains $\power2{i-1}$ vertexes
\begin{justification}[method=byIH](IH)\end{justification}
\end{spfstep}
\begin{spfstep}
By the \begin{justification}[method=byDef]definition of a binary