TeX/ifvoid

Synopsis
\ifvoid<0..255>  [\else ] \fi

Description
The \ifvoid command denotes the start of an if-then-else control structure, where <0..255> must expand to an integer from 0 to 255. If the box register \box<0..255> is void, that is contains nothing, then  is processed and  is ignored, if the \else section is included. If \box<0..255> contains a box then  is ignored and  is processed, if the \else section is included.