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?