Testing whether a document has option "fleqn"

Hefferon, Jim S. jhefferon at smcvt.edu
Sat May 4 13:49:40 CEST 2019

Thank you, Herbert.  I should have thought of that.  However ..

    \trivlist \item \relax \hspace*{\mathindent}
    \trivlist \centering \item \relax
  \endtrivlist %

still gives me 

! LaTeX Error: Can be used only in preamble.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
l.1582 \begin{display}


... those dreadful hammers!
  --John Ruskin, 1851

From: Herbert Voss <Herbert.Voss at fu-berlin.de>
Sent: Saturday, May 4, 2019 0:14
To: texhax at tug.org
Subject: Re: Testing whether a document has option "fleqn"

Am 04.05.19 um 00:30 schrieb Hefferon, Jim S.:
> How can I test whether the document has the class option `fleqn`?  I want to have an environment that takes the place of center but that if required gives content that is left-aligned, as equations are left-aligned under fleqn.
> I had in mind something like this.  However this one gives `! LaTeX Error: Can be used only in preamble.`
> \newenvironment{display}{%
>     \@ifclasswith{fleqn}{
>        \trivlist \item \relax \hspace*{\mathindent}
>        }{
>        \trivlist \centering \item \relax
>      }}{
>      \endtrivlist
>      }
> I know that there are flags like \@fleqntrue and have spent some time trying toggle commands, etc, but been unable to hit on the right one.  A tip would really help me.


             \trivlist \item \relax \hspace*{\mathindent}
             \trivlist \centering \item \relax


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