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Lars wrote

> 2. Sometimes, an existing command is redefined so that an optional
> argument is added at the end. In such cases, it may be preferable to make
> that argument such that it does not skip spaces, for compatibility with
> existing documents written under the assumption that spaces would not be
> skipped at that point. In other words, if
>
>     \bar{apa} cepa
>
> used to have a space, and \bar for some reason in a package needs to be
> extended to support \bar{apa}[bepa], then \bar{apa} cepa should still have
> that space.

in this particular case it would indeed be a bad change if a package "extends"
a command in this way and the result is the loss of spaces in a document.
On the other hand I don't think that a package author doing this would get
very happy customers

> Therefore, the solution should be to provide both, but let the
> space-nonskipping variants come with a big warning in the xparse
> documentation, detailing why they are usually inappropriate. In
> particular, such documentation should suggest the argument order
>
>    \bar[bepa]{apa}
>
> as preferable to
>
>    \bar{apa}[bepa]
>
> since package authors are otherwise likely to pick one at random, never
> even considering the syntactical implications.

This might be an option for a compromise (in fact you could even force this to
be only available in the last argument at some technical cost, though I would
advice to not go that far but put in big guidelines)

something  to think about a bit further I guess (but not tonight)

frank