An mo element represents an operator or anything that should be rendered as an operator. In MathML the list of things that should "render as an operator" includes a number of notations that are not mathematical operators in the ordinary sense. Besides ordinary operators with infix, prefix, or postfix forms, these include fence characters such as braces, parentheses, and "absolute value" bars; separators such as comma and semicolon; and mathematical accents such as a bar or tilde over a symbol. MathML Fundamentals
<math display="block"> <mspace linebreak="newline" indentalign="center"></mspace> <mover style="border:1px"> <mrow style="border:5px"> <mi>very long</mi> <mo>+</mo> <mo>...</mo> <mo>+</mo> <mi>texttttttttttttt</mi> </mrow> <mover style="border:1px"> <mo style="border:2px">⏞</mo> <mrow><mi>k</mi> <mspace width="thinmathspace"></mspace> <mtext>times</mtext></mrow> </mover> </mover> <mspace linebreak="newline" indentalign="center"></mspace> <mover style="border:1px"> <mrow style="border:5px"> <mi>very long</mi> <mo>+</mo> <mo>...</mo> <mo>+</mo> <mi>texttttttttttttt</mi> </mrow> <munder style="border:1px"> <mo style="border:2px">⏞</mo> <mrow><mi>k</mi> <mspace width="thinmathspace"></mspace> <mtext>times</mtext></mrow> </munder> </mover> <mspace linebreak="newline" indentalign="center"></mspace> <mover style="border:1px"> <mrow style="border:5px"> <mi>very long</mi> <mo>+</mo> <mo>...</mo> <mo>+</mo> <mi>texttttttttttttt</mi> </mrow> <munderover style="border:1px"> <mo style="border:2px">⏞</mo> <mrow><mi>k</mi> <mspace width="thinmathspace"></mspace> <mtext>times</mtext></mrow> <mi>k</mi> </munderover> </mover> <mspace linebreak="newline" indentalign="center"></mspace> <mover style="border:1px"> <mrow style="border:5px"> <mi>very long</mi> <mo>+</mo> <mo>...</mo> <mo>+</mo> <mi>texttttttttttttt</mi> </mrow> <msup style="border:1px"> <mo style="border:2px" >⏞</mo> <mi style="border:1px">2</mi> </msup> </mover> <mspace linebreak="newline" indentalign="center"></mspace> <mover style="border:1px"> <mrow style="border:5px"> <mi>very long</mi> <mo>+</mo> <mo>...</mo> <mo>+</mo> <mi>texttttttttttttt</mi> </mrow> <msub style="border:1px"> <mo style="border:2px" >⏞</mo> <mi style="border:1px">23</mi> </msub> </mover> <mspace linebreak="newline" indentalign="center"></mspace> <mover style="border:1px"> <mrow style="border:5px"> <mi>very long</mi> <mo>+</mo> <mo>...</mo> <mo>+</mo> <mi>texttttttttttttt</mi> </mrow> <msubsup style="border:1px"> <mo style="border:2px" >⏞</mo> <mi style="border:1px">2</mi> <mi style="border:1px">233</mi> </msubsup> </mover> <mspace linebreak="newline" indentalign="center"></mspace> <mover style="border:1px"> <mrow style="border:5px"> <mi>very long</mi> <mo>+</mo> <mo>...</mo> <mo>+</mo> <mi>texttttttttttttt</mi> </mrow> <msubsup style="border:1px"> <mo style="border:2px" >⏞</mo> <mi style="border:1px">233</mi> <mi style="border:1px">2</mi> </msubsup> </mover> <mspace linebreak="newline" indentalign="center"></mspace> <mover style="border:1px"> <mrow style="border:5px"> <mi>very long</mi> <mo>+</mo> <mo>...</mo> <mo>+</mo> <mi>texttttttttttttt</mi> </mrow> <mmultiscripts style="border:1px"> <mo style="border:2px" >⏞</mo> <mi style="border:1px">i</mi> <none/> <none/> <mi style="border:1px">j</mi> <mi style="border:1px">k</mi> <none/> <mi style="border:1px">l</mi> <none/> <mprescripts/> <mn style="border:1px">0</mn> <mn style="border:1px">12</mn> <mn style="border:1px">12</mn> <none/> </mmultiscripts> </mover> <mspace linebreak="newline" indentalign="center"></mspace> <mover style="border:1px"> <mrow style="border:5px"> <mi>very long</mi> <mo>+</mo> <mo>...</mo> <mo>+</mo> <mi>texttttttttttttt</mi> </mrow> <mrow style="border:1px"> <mi>very</mi> <mo>+</mo> <mo style="border:2px" >→</mo> <mo>+</mo> <mi>text</mi> </mrow> </mover> <mspace linebreak="newline" indentalign="center"></mspace> <mover style="border:1px"> <mrow style="border:5px"> <mi>very long</mi> <mo>+</mo> <mo>...</mo> <mo>+</mo> <mi>texttttttttttttt</mi> </mrow> <mrow style="border:1px"> <mi>x</mi> <mo>+</mo> <mo style="border:2px" >→</mo> <mo>+</mo> <mo style="border:2px" >→</mo> <mo>+</mo> <mo style="border:2px" >→</mo> <mo>+</mo> <mi>y</mi> </mrow> </mover> <mspace linebreak="newline" indentalign="center"></mspace> <mover style="border:1px"> <mrow style="border:5px"> <mi>very long</mi> <mo>+</mo> <mo>...</mo> <mo>+</mo> <mi>texttttttttttttt</mi> </mrow> <mrow style="border:1px"> <mi>x</mi> <mo>+</mo> <mo style="border:2px" >→</mo> <mo>+</mo> <mo style="border:2px" stretchy="false">→</mo> <mo>+</mo> <mo style="border:2px" >→</mo> <mo>+</mo> <mi>y</mi> </mrow> </mover> <mspace linebreak="newline" indentalign="center"></mspace> <mover style="border:1px"> <mrow style="border:5px"> <mi>very long</mi> <mo>+</mo> <mo>...</mo> <mo>+</mo> <mi>texttttttttttttt</mi> </mrow> <mrow style="border:1px"> <mrow style="border:1px"> <mi>x</mi> <mo>+</mo> <mo style="border:2px" >→</mo> <mo>+</mo> <mo style="border:2px" stretchy="false">→</mo> <mo>+</mo> <mo style="border:2px" >→</mo> <mo>+</mo> <mi>y</mi> </mrow> </mrow> </mover> </math>