\begin{funcdesc}{collect}{\optional{generation}}
With no arguments, run a full collection. The optional argument
\var{generation} may be an integer specifying which generation to collect
-(from 0 to 2). A ValueError is raised if the generation number is invalid.
+(from 0 to 2). A \exception{ValueError} is raised if the generation number
+is invalid.
The number of unreachable objects found is returned.
\versionchanged[The optional \var{generation} argument was added]{2.5}