[texhax] Trying to find if a listing is continued with listings package

[Philip Taylor]

P.S.  I think the following demonstrates why \@undefined is markedly
inferior to \ifcsname :

> \documentclass {minimal}
> \catcode `\@ = \catcode `\a
> \begin {document}
> \ifcsname 42\endcsname \message {42: Known csname}\else \message {42: UNKnown csname}\fi
> \message {; }
> \@ifundefined {42}{\message {42: UNdefined}}{\message {42: defined}}
> \message {; }
> \ifcsname 42\endcsname \message {42: Known csname}\else \message {42: UNKnown csname}\fi
> \message {.}
> \end {document}

This outputs

	42: UNKnown csname;
	42: UNdefined;
	42: Known csname.

whereas the following (using \ifcsname)

> \documentclass {minimal}
> \catcode `\@ = \catcode `\a
> \begin {document}
> \ifcsname 42\endcsname \message {42: Known csname}\else \message {42: UNKnown csname}\fi
> \message {; }
> \unless \ifcsname 42\endcsname \message {42: UNdefined}\else \message {42: defined}\fi
> \message {; }
> \ifcsname 42\endcsname \message {42: Known csname}\else \message {42: UNKnown csname}\fi
> \message {.}
> \end {document}

outputs

> 42: UNKnown csname;
> 42: UNdefined;
> 42: UNKnown csname.

Thus \@undefined causes something that was previously undefined to
become defined; \ifcsname does not.

** Phil.