rec-domain1

W3C sample	WIRIS
	$\mathop{\mathrm{domain}}(f)=\mathbb{R}$

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <apply>
    <eq/>
    <apply>
      <domain/>
      <fn>
        <ci>f</ci>
      </fn>
    </apply>
    <reals/>
  </apply>
</math>

Author: MathML 2 Recommendation