\begin{tabbing}
{\rm -}{\rm -}\xspa{1}{\com list\/}\xspa{1}{\com concatenation\/}\xspa{1}$(${\com right\/}$-${\com associative\/}$)$\\
\begin{tabular}{@{}l@{\xspa1}c@{}l}
$($\plusplus$)$\xspa{20} & $::$ & \xspa{1}$[${\iden a\/}$]$\xspa{1}$\rightarrow$\xspa{1}$[${\iden a\/}$]$\xspa{1}$\rightarrow$\xspa{1}$[${\iden a\/}$]$\\
{\iden xs\/}\xspa{1}\plusplus\xspa{1}{\iden ys\/}\xspa{16} & $=$ & \xspa{2}{\iden foldr\/}\xspa{1}$(:)$\xspa{1}{\iden ys\/}\xspa{1}{\iden xs\/}
\end{tabular}
\end{tabbing}
