Content markup for conditional expression

Hi, everyone:

I have a question for the content markup: how to markup a conditional
expression such as:

      f1(x)      x > 0
y = { f2(x)      x = 0
      fn(x)      otherwise

By reading through MathML, I can't find a way of doing it. But this kind of
expression is very basic in mathematics I believe.

I looked at the example in the software EzMath, where special markup tags
<ea> and <st> are used and they can not be found in MathML.

Can some one help me to answer this question?

Jian Li
Physiome Sciences, Inc.