Note that the expression ``$x$ and $y$ are $k$\nbd{}composable'' is \emph{not} symmetric in $x$ and $y$ and we \emph{should} rather speak of a ``$k$\nbd{}composable pair $(x,y)$'', although we won't always do it. The set of pairs of $k$\nbd{}composable $n$\nbd{}cells is denoted by $X_n\underset{X_k}{\times}X_n$, and is characterized as the following fibred product
Note that the expression \og$x$ and $y$ are $k$\nbd{}composable\fg{} is \emph{not} symmetric in $x$ and $y$ and we \emph{should} rather speak of a ``$k$\nbd{}composable pair $(x,y)$'', although we won't always do it. The set of pairs of $k$\nbd{}composable $n$\nbd{}cells is denoted by $X_n\underset{X_k}{\times}X_n$, and is characterized as the following fibred product
\[
\begin{tikzcd}
X_n\underset{X_k}{\times}X_n \ar[r]\ar[dr,phantom,"\lrcorner", very near start]\ar[d]&X_n \ar[d,"\trgt_k"]\\