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\ifvoid<0..255> <tex-code-1> [\else <tex-code-2>] \fi


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 <tex-code-1> is processed and <tex-code-2> is ignored, if the \else section is included. If \box<0..255> contains a box then <tex-code-1> is ignored and <tex-code-2> is processed, if the \else section is included.