MuparserX is not thread-safe because of its use of static variables:
https://github.com/beltoforion/muparserx/blob/f014908f1e3efdb230eade0296a175dc04d0d1b3/parser/mpIToken.h#L127
https://github.com/beltoforion/muparserx/blob/f014908f1e3efdb230eade0296a175dc04d0d1b3/parser/mpPackageStr.h#L58
and several other locations.