Several of the predefined solids have a ``\textit{hollow}'' relative which is naturally associated with it (the cone, the truncated cone, the cylinder, the prism and the spherical zone). For all those, the option \texttt{\Lkeyword{hollow}=true} is provided.
Set to \texttt{false}, we get the ``filled'' solid; set to \texttt{true} we get the ``hollow'' version.
\subsubsection{Example 1: a \Index{cylinder} and a \Index{hollow cylinder}}