A double-symbol default argument in environment. Is this a known bug?

Question

A question inspired by @gernot's elegant answer in the topic Environment with a dummy parameter.

However some further experiments lead to the following. Indeed, what the devil? Take a @gernot version of the look-environment there and look at this.

\documentclass{article}
\begin{document}

\newenvironment{look}[1][\empty]%
{\begin{trivlist}\item[]\textsc{ThinkOut}% 
\ifx#1\empty\else\ (#1)\fi.\itshape}{\end{trivlist}}

\begin{verbatim}
\begin{look}[some addendum]
Text ...
\end{look}
\end{verbatim}
\begin{look}[some addendum]
Text ...
\end{look}

\begin{verbatim}
\begin{look}[q]
Seems allright? Look below ...
\end{look}
\end{verbatim}
\begin{look}[q]
Seems allright? Look below ...
\end{look}

\begin{verbatim}
\begin{look}[qq]
Double-q disappears. Where parentheses?
\end{look}
\end{verbatim}
\begin{look}[qq]
Double-q disappears. Where parentheses?
\end{look}

\begin{verbatim}
\begin{look}[aaa]
One of three a's survives; no ()'s do.
\end{look}
\end{verbatim}
\begin{look}[aaa]
One of three a's survives; no ()'s do.
\end{look}

\begin{verbatim}
\begin{look}[ qq]
Allright again.
\end{look}
\end{verbatim}
\begin{look}[ qq]
Allright again.
\end{look}

\begin{verbatim}
\begin{look}[{}aa]
Allright too.
\end{look}
\end{verbatim}
\begin{look}[{}aa]
Allright too.
\end{look}

\begin{verbatim}
\begin{look}[==]
Any double symbol disappears.
\end{look}
\end{verbatim}
\begin{look}[==]
Any double symbol disappears.
\end{look}

\end{document}

Or may be this is the known LaTeX bug?

PS. Sorry, I was not able to download the pdf-result file here.

PPS. The system I have is (info from a log-file)

e-TeX, Version 3.141592-2.1 (MiKTeX 2.3) (preloaded format=latex 2000.11.28)

LaTeX2e <2001/06/01> Document Class: article 2001/04/21 v1.4e

Standard LaTeX document class

Answer 1

No bug at all: with \begin{look}[qq] the test is

\ifx qq\empty

which obviously evaluates true.

The test should be

\ifx\empty#1\relax

but I'd prefer

\newenvironment{look}[1][]
  {%
   \begin{trivlist}
   \item\relax\textsc{ThinkOut}% 
   \if\relax\detokenize{#1}\relax
   \else
     \ (#1)%
   \fi.
   \itshape
  }
  {\end{trivlist}}

---

What happens with your version and \begin{look}[ab]?

The code does

\ifx ab\empty\else\ (ab)\fi

Since the test evaluates false, the false branch is followed. If you do \begin{look}[abcdefg], the test is

\ifx abcdefg\empty\else\ (abcdefg)\fi

and the tokens from c to \empty disappear as part of the true branch. If you try \begin{look}[aabbcc], the test is

\ifx aabbcc\empty\else\ (aabbcc)\fi

and you get bbcc printed.