\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