Currently, the SBML export may include <true> and <false> tags, which are not valid MathML (I think?). At least they are not supported by (our) parser -- oops.
It seems that constants are indeed supported by MathML (and SBML as well). This is also fixed in the parser now using ca0ba4b9c1dc89d128f674792fb681a0255a0f1d.
Currently, the SBML export may include
<true>
and<false>
tags, which are not valid MathML (I think?). At least they are not supported by (our) parser -- oops.