Testing whether a document has option "fleqn"

[Herbert Voss]

Am 04.05.19 um 13:49 schrieb Hefferon, Jim S.:
> \AtBeginDocument{\newenvironment{display}{%
>    \@ifclasswith{fleqn}{%
>      \trivlist \item \relax \hspace*{\mathindent}
>      }{%
>      \trivlist \centering \item \relax
>      }
> }{%
>    \endtrivlist %
> }}
> 
> still gives me
> 
> ! LaTeX Error: Can be used only in preamble.

Jim,

can you provide a short complete example?

Best
Herbert