Hi Bruce (and Morten as, I think, the prime suspect re math style)
This is not an answer but I think I see what has happened.
amsmath has an internal command \@genfac and this is what has been used, in some sense, as the basis for the user-command in mathstyle.
I refrain from comment of whether this change was a Good Thing (™).
It could be that the required contents of the 'style' parameter are not compatible so more work may be needed than a change of parameter ordering.
\@genfrac takes the preceding arguments and reads the numerator and de- nominator. Note that there’s no convenient way to make the numerator and denominator contents displaystyle, through this interface.
Args: #1 mathstyle, #2 fraction primitive, #3 delimiters and rule thickness, #4 numerator, #5 denominator.
Sent from my iPud (\~/)
> On 28 Oct 2014, at 17:29, Bruce Miller <[log in to unmask]> wrote:
> Hi all;
> Not sure whether this is the right place for this
> message, but a colleague is using breqn, which
> pulls in a lot of l3kernel stuff, and also mathstyle.
> Unfortunately, it appears that the package mathstyle
> redefines \genfrac with a different argument order
> than amsmath. (and worse, wants to be loaded after
> amsmath, so you're stuck with the redefinition)
> mathstyle's api seems to be:
> while amsmath's is
> Is there a good reason for this?