<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <semantics>
    <mo>+</mo>
    <annotation-xml encoding="MathML-Content">
      <plus/>
    </annotation-xml>
    <annotation encoding="SnuggleTeX">\[ + \]</annotation>
    <annotation encoding="LaTeX">\[ + \]</annotation>
    <annotation encoding="Maxima">operator("+")</annotation>
  </semantics>
</math>

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <semantics>
    <mo>+</mo>
    <annotation encoding="SnuggleTeX">\[ + \]</annotation>
  </semantics>
</math>

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mo>+</mo>
</math>

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <plus/>
</math>

operator("+")