rec-declare2

W3C sample WIRIS
V 1 2 3 
<math xmlns="http://www.w3.org/1998/Math/MathML">
  <declare type="vector">
    <ci>V</ci>
    <vector>
      <cn>1</cn>
      <cn>2</cn>
      <cn>3</cn>
    </vector>
  </declare>
</math>

Author: MathML 2 Recommendation