convert the empty set without using a special case

[MarcoCostantini]

The empty set is converted from Python to OpenMath as
<OMS cdbase="http://www.openmath.org/cd" name="emptyset" cd="set1"/>
which is correct but not the best choice. It would be better to convert it as
<OMA><OMS cdbase=\"http://www.openmath.org/cd\" cd=\"set1\" name=\"set\"/></OMA>
it such way, the empty set is handled as a set and not as a special case.

About this, please consider what is said in
http://www.catb.org/~esr/writings/taoup/html/ch01s06.html , which is
especially relevant for OpenMath, which is intended to be used by
computers:

For robustness, designing in tolerance for unusual or extremely bulky
inputs is also important. Bearing in mind the Rule of Composition helps;
input generated by other programs is notorious for stress-testing software
(e.g., the original Unix C compiler
reportedly needed small upgrades to cope well with Yacc output). The forms
involved often seem useless to humans. For example, accepting empty
lists/strings/etc., even in places where a human would seldom or never
supply an empty string, avoids having to
special-case such situations when generating the input mechanically.
-- Henry Spencer
One very important tactic for being robust under odd inputs is to avoid
having special cases in your code. Bugs often lurk in the code for
handling special cases, and in the interactions among parts of the code
intended to handle different special cases.