holtzermann17
commented
10 years ago @rspuzio There was some old code in the problem module that mas mangling things. I removed that code, revised the problem you added, and it works fine now.
BTW you also needed \usepackage{xspace} in that preamble to give the \xspace command (added).
I just added the following problem to PM:
http://planetmath.org/node/87531
It requires a tiny preamble to make it work:
\newcommand{\eqv}[2]{\ensuremath{#1 \simeq #2}\xspace}
However, once I save it, it gives a big red \eqv and, when I go to edit it again, the preamble disappears and is replaced with the inane message
%SITEWIDE DEFAULT SHOULD GO HERE
A long time ago, we had this problem with articles as the now-closed issue #157 "address disappearing preamble problems". While it was fixed for entries back then, it looks like it was overlooked or has reappeared for problems; presumably, it is a matter of applying the same fix again.