Mathml in sgml environment

we're currently looking for a mechanism to implement maths 
in our sgml data.
We want to make use of mathml for that.
I had troubles including mathml.dtd as a module in existing sgml dtd's?
At a certain point in my sgml dtd I want to include mathml.

<!element formula - - (math , p)
Element math being part of the mathml.dtd.

I got mathml.dtd errors when I tried this.

So, in general how can you make us of xml fragments in a
sgml dtd environment?
Another sgml declaration? Changing to xml dtd syntax to be sgml