bug#25447: 11.90.0; alltt environment should allow being extended in wrapper

[jfbu]

Hi,

\newenvironment{myalltt}
{\begin{alltt}%
   \IamAllowedToDefineWrapperOfAllTT
   \ButFontificationByAucTeXIsProblematic
  }
{\end{alltt}}

causes fontification problem. I can use \alltt and \endalltt
but this forces me to change TeX code for this issue.

(alltt is not implemented like standard verbatim, the latter
would indeed not allow the above kind of redefinition)

Best,

Jean-François