rec-selector1

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <apply>
    <csymbol cd="linagl1">vector_selector</csymbol>
    <cn>1</cn>
    <apply>
      <csymbol cd="linalg2">matrix</csymbol>
      <apply>
        <csymbol cd="linalg2">matrixrow</csymbol>
        <cn>1</cn>
        <cn>2</cn>
      </apply>
      <apply>
        <csymbol cd="linalg2">matrixrow</csymbol>
        <cn>3</cn>
        <cn>4</cn>
      </apply>
    </apply>
  </apply>
</math>

Author: MathML 2 Recomendation